diff --git a/dom/base/test/bug457746.sjs b/dom/base/test/bug457746.sjs
index d014df2cee06..02135c0a5c23 100644
--- a/dom/base/test/bug457746.sjs
+++ b/dom/base/test/bug457746.sjs
@@ -6,6 +6,6 @@ function handleRequest(request, response)
       .createInstance(Components.interfaces.nsIBinaryOutputStream);
   bos.setOutputStream(response.bodyOutputStream);
 
-  bos.writeByteArray(body, body.length);
+  bos.writeByteArray(body);
 }
 
diff --git a/dom/events/DataTransfer.cpp b/dom/events/DataTransfer.cpp
index 1937ab223e11..13d490d29ac5 100644
--- a/dom/events/DataTransfer.cpp
+++ b/dom/events/DataTransfer.cpp
@@ -7,6 +7,7 @@
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/BasicEvents.h"
 #include "mozilla/CheckedInt.h"
+#include "mozilla/Span.h"
 
 #include "DataTransfer.h"
 
@@ -1104,8 +1105,12 @@ already_AddRefed<nsITransferable> DataTransfer::GetTransferable(
                 totalCustomLength = 0;
                 continue;
               }
-              rv = stream->WriteBytes((const char*)type.get(),
-                                      formatLength.value());
+              MOZ_ASSERT(formatLength.isValid() &&
+                             formatLength.value() ==
+                                 type.Length() * sizeof(nsString::char_type),
+                         "Why is formatLength off?");
+              rv = stream->WriteBytes(
+                  AsBytes(MakeSpan(type.BeginReading(), type.Length())));
               if (NS_WARN_IF(NS_FAILED(rv))) {
                 totalCustomLength = 0;
                 continue;
@@ -1115,7 +1120,13 @@ already_AddRefed<nsITransferable> DataTransfer::GetTransferable(
                 totalCustomLength = 0;
                 continue;
               }
-              rv = stream->WriteBytes((const char*)data.get(), lengthInBytes);
+              // XXXbz it's not obvious to me that lengthInBytes is the actual
+              // length of "data" if the variant contained an nsISupportsString
+              // as VTYPE_INTERFACE, say.  We used lengthInBytes above for
+              // sizing, so just keep doing that.
+              rv = stream->WriteBytes(MakeSpan(
+                  reinterpret_cast<const uint8_t*>(data.BeginReading()),
+                  lengthInBytes));
               if (NS_WARN_IF(NS_FAILED(rv))) {
                 totalCustomLength = 0;
                 continue;
diff --git a/dom/ipc/tests/blob_verify.sjs b/dom/ipc/tests/blob_verify.sjs
index 62b82359e593..cf50371c8a95 100644
--- a/dom/ipc/tests/blob_verify.sjs
+++ b/dom/ipc/tests/blob_verify.sjs
@@ -15,6 +15,6 @@ function handleRequest(request, response) {
   var bos = new BinaryOutputStream(response.bodyOutputStream);
 
   response.processAsync();
-  bos.writeByteArray(bodyBytes, bodyBytes.length);
+  bos.writeByteArray(bodyBytes);
   response.finish();
 }
diff --git a/dom/serviceworkers/test/fetch/deliver-gzip.sjs b/dom/serviceworkers/test/fetch/deliver-gzip.sjs
index abacdd2adb8c..30c9e69ce7e1 100644
--- a/dom/serviceworkers/test/fetch/deliver-gzip.sjs
+++ b/dom/serviceworkers/test/fetch/deliver-gzip.sjs
@@ -13,5 +13,5 @@ function handleRequest(request, response) {
       .createInstance(Components.interfaces.nsIBinaryOutputStream);
   bos.setOutputStream(response.bodyOutputStream);
 
-  bos.writeByteArray(bytes, bytes.length);
+  bos.writeByteArray(bytes);
 }
diff --git a/dom/xhr/tests/temporaryFileBlob.sjs b/dom/xhr/tests/temporaryFileBlob.sjs
index ff3410c78c8a..814c06d1ed4e 100644
--- a/dom/xhr/tests/temporaryFileBlob.sjs
+++ b/dom/xhr/tests/temporaryFileBlob.sjs
@@ -21,10 +21,10 @@ function handleRequest(request, response) {
   response.processAsync();
 
   var part = bodyBytes.splice(0, 256);
-  bos.writeByteArray(part, part.length);
+  bos.writeByteArray(part);
 
   response.timer1 = new Timer(function(timer) {
-    bos.writeByteArray(bodyBytes, bodyBytes.length);
+    bos.writeByteArray(bodyBytes);
   }, 1000, Components.interfaces.nsITimer.TYPE_ONE_SHOT);
 
   response.timer2 = new Timer(function(timer) {
diff --git a/image/test/reftest/generic/check-header.sjs b/image/test/reftest/generic/check-header.sjs
index 9e905bc1cf0d..afc39cb550b4 100644
--- a/image/test/reftest/generic/check-header.sjs
+++ b/image/test/reftest/generic/check-header.sjs
@@ -62,7 +62,7 @@ function handleRequest(request, response)
     response.setHeader("Content-Type", "image/png", false);
 
     var stream = new BinaryOutputStream(response.bodyOutputStream);
-    stream.writeByteArray(IMAGE_DATA, IMAGE_DATA.length);
+    stream.writeByteArray(IMAGE_DATA);
   }
   else
   {
diff --git a/image/test/unit/test_imgtools.js b/image/test/unit/test_imgtools.js
index eaab4b915ff9..89810d22b42d 100644
--- a/image/test/unit/test_imgtools.js
+++ b/image/test/unit/test_imgtools.js
@@ -25,7 +25,7 @@ function dumpToFile(aData) {
   );
   bos.setOutputStream(outputStream);
 
-  bos.writeByteArray(aData, aData.length);
+  bos.writeByteArray(aData);
 
   outputStream.close();
 }
diff --git a/intl/uconv/tests/unit/test_charset_conversion.js b/intl/uconv/tests/unit/test_charset_conversion.js
index e32656f9160f..2b800bfce470 100644
--- a/intl/uconv/tests/unit/test_charset_conversion.js
+++ b/intl/uconv/tests/unit/test_charset_conversion.js
@@ -89,7 +89,7 @@ function test_cross_conversion()
     while ((av = fin.available()) > 0)
     {
       var data = fin.readByteArray(av);
-      bos.writeByteArray(data, data.length);
+      bos.writeByteArray(data);
     }
     fin.close();
     bos.close();
diff --git a/js/xpconnect/src/nsXPConnect.cpp b/js/xpconnect/src/nsXPConnect.cpp
index b4bb8d1b4460..794d2066a171 100644
--- a/js/xpconnect/src/nsXPConnect.cpp
+++ b/js/xpconnect/src/nsXPConnect.cpp
@@ -973,7 +973,8 @@ static nsresult WriteScriptOrFunction(nsIObjectOutputStream* stream,
   }
   rv = stream->Write32(size);
   if (NS_SUCCEEDED(rv)) {
-    rv = stream->WriteBytes(reinterpret_cast<char*>(buffer.begin()), size);
+    // Ideally we could just pass "buffer" here.  See bug 1566574.
+    rv = stream->WriteBytes(MakeSpan(buffer.begin(), size));
   }
 
   return rv;
diff --git a/layout/reftests/backgrounds/background-referrer.sjs b/layout/reftests/backgrounds/background-referrer.sjs
index 7ead6e20efea..a00a71ebd844 100644
--- a/layout/reftests/backgrounds/background-referrer.sjs
+++ b/layout/reftests/backgrounds/background-referrer.sjs
@@ -62,7 +62,7 @@ function handleRequest(request, response)
     response.setHeader("Content-Type", "image/png", false);
 
     var stream = new BinaryOutputStream(response.bodyOutputStream);
-    stream.writeByteArray(IMAGE_DATA, IMAGE_DATA.length);
+    stream.writeByteArray(IMAGE_DATA);
   }
   else
   {
diff --git a/layout/reftests/backgrounds/delay-image-response.sjs b/layout/reftests/backgrounds/delay-image-response.sjs
index f3117f8d5452..418eaa24a976 100644
--- a/layout/reftests/backgrounds/delay-image-response.sjs
+++ b/layout/reftests/backgrounds/delay-image-response.sjs
@@ -43,7 +43,7 @@ function handleRequest(request, response) {
 
   function imageWrite() {
     var stream = new BinaryOutputStream(response.bodyOutputStream);
-    stream.writeByteArray(IMAGE_DATA, IMAGE_DATA.length);
+    stream.writeByteArray(IMAGE_DATA);
   }
 
   // If there is no delay, we write the image and leave.
diff --git a/layout/reftests/font-face/bug-1481905-cancel-load.sjs b/layout/reftests/font-face/bug-1481905-cancel-load.sjs
index 85f8b89fc8ba..69c650217719 100644
--- a/layout/reftests/font-face/bug-1481905-cancel-load.sjs
+++ b/layout/reftests/font-face/bug-1481905-cancel-load.sjs
@@ -205,7 +205,7 @@ function handleRequest(request, response) {
 
   function fontWrite(data) {
     var stream = new BinaryOutputStream(response.bodyOutputStream);
-    stream.writeByteArray(data, data.length);
+    stream.writeByteArray(data);
   }
 
   const nsITimer = Components.interfaces.nsITimer;
diff --git a/layout/reftests/fonts/Chunkfive.sjs b/layout/reftests/fonts/Chunkfive.sjs
index fe3077cc598d..fe09ccbe5fb7 100644
--- a/layout/reftests/fonts/Chunkfive.sjs
+++ b/layout/reftests/fonts/Chunkfive.sjs
@@ -19,7 +19,7 @@ function handleRequest(request, response)
   response.setHeader("Cache-Control", "no-cache", false);
 
   var stream = new BinaryOutputStream(response.bodyOutputStream);
-  stream.writeByteArray(FONT_DATA, FONT_DATA.length);
+  stream.writeByteArray(FONT_DATA);
 }
 
 /**
diff --git a/layout/reftests/fonts/markfonts-delay.sjs b/layout/reftests/fonts/markfonts-delay.sjs
index 574b1404dc21..1d3a43e47c22 100644
--- a/layout/reftests/fonts/markfonts-delay.sjs
+++ b/layout/reftests/fonts/markfonts-delay.sjs
@@ -37,7 +37,7 @@ function handleRequest(request, response) {
 
   function fontWrite(data) {
     var stream = new BinaryOutputStream(response.bodyOutputStream);
-    stream.writeByteArray(data, data.length);
+    stream.writeByteArray(data);
   }
 
   var f;
diff --git a/layout/style/test/redundant_font_download.sjs b/layout/style/test/redundant_font_download.sjs
index 7eb7d8f8df54..a28fc081d24d 100644
--- a/layout/style/test/redundant_font_download.sjs
+++ b/layout/style/test/redundant_font_download.sjs
@@ -39,11 +39,11 @@ function handleRequest(request, response)
   if (query["q"] == "init") {
     log = "init"; // initialize the log, and return a PNG image
     response.setHeader("Content-Type", "image/png", false);
-    stream.writeByteArray(RED_SQUARE, RED_SQUARE.length);
+    stream.writeByteArray(RED_SQUARE);
   } else if (query["q"] == "image") {
     log = log + ";" + query["q"];
     response.setHeader("Content-Type", "image/png", false);
-    stream.writeByteArray(RED_SQUARE, RED_SQUARE.length);
+    stream.writeByteArray(RED_SQUARE);
   } else if (query["q"] == "font") {
     log = log + ";" + query["q"];
     // we don't provide a real font; that's ok, OTS will just reject it
diff --git a/netwerk/test/httpserver/httpd.js b/netwerk/test/httpserver/httpd.js
index 777a99e0f41c..95f39cf604b6 100644
--- a/netwerk/test/httpserver/httpd.js
+++ b/netwerk/test/httpserver/httpd.js
@@ -1532,9 +1532,10 @@ RequestReader.prototype = {
             " excess=" +
             (data.length - count)
         );
+        data.length = count;
 
         var bos = new BinaryOutputStream(this._metadata._bodyOutputStream);
-        bos.writeByteArray(data, count);
+        bos.writeByteArray(data);
         this._contentLength -= count;
       }
 
@@ -2861,7 +2862,7 @@ ServerHandler.prototype = {
                 ", expected " +
                 chunkSize
             );
-            output.writeByteArray(data, data.length);
+            output.writeByteArray(data);
             if (count === 0) {
               fis.close();
               response.finish();
diff --git a/netwerk/test/httpserver/test/test_async_response_sending.js b/netwerk/test/httpserver/test/test_async_response_sending.js
index 0013d2e9b54d..780f3e1a41a4 100644
--- a/netwerk/test/httpserver/test/test_async_response_sending.js
+++ b/netwerk/test/httpserver/test/test_async_response_sending.js
@@ -867,19 +867,9 @@ function CustomPipe(name) {
     //
     // see nsIBinaryOutputStream.writeByteArray
     //
-    writeByteArray: function writeByteArray(bytes, length) {
-      dumpn(
-        "*** [" +
-          this.name +
-          "].writeByteArray" +
-          "([" +
-          bytes +
-          "], " +
-          length +
-          ")"
-      );
+    writeByteArray: function writeByteArray(bytes) {
+      dumpn(`*** [${this.name}].writeByteArray([${bytes}])`);
 
-      Assert.equal(bytes.length, length, "sanity");
       if (!Components.isSuccessCode(self._status)) {
         throw self._status;
       }
@@ -890,12 +880,12 @@ function CustomPipe(name) {
         "writeByteArray can't support specified-length writes"
       );
 
-      if (this._writable < length) {
+      if (this._writable < bytes.length) {
         throw Cr.NS_BASE_STREAM_WOULD_BLOCK;
       }
 
       self._data.push.apply(self._data, bytes);
-      this._writable -= length;
+      this._writable -= bytes.length;
 
       if (
         input._readable === Infinity &&
@@ -1164,7 +1154,7 @@ CopyTest.prototype = {
 
       try {
         self._copyableDataStream.makeWritable(bytes.length);
-        self._copyableDataStream.writeByteArray(bytes, bytes.length);
+        self._copyableDataStream.writeByteArray(bytes);
       } finally {
         self._stageNextTask();
       }
diff --git a/netwerk/test/httpserver/test/test_processasync.js b/netwerk/test/httpserver/test/test_processasync.js
index 3effd1ec1570..33bc10069820 100644
--- a/netwerk/test/httpserver/test/test_processasync.js
+++ b/netwerk/test/httpserver/test/test_processasync.js
@@ -240,7 +240,7 @@ function handleAsyncOrdering(request, response) {
       }
 
       try {
-        out.writeByteArray(data, data.length);
+        out.writeByteArray(data);
         step();
       } catch (e) {
         try {
diff --git a/netwerk/test/unit/test_bug1279246.js b/netwerk/test/unit/test_bug1279246.js
index c33f5c2e0970..b3742b1e2bd7 100644
--- a/netwerk/test/unit/test_bug1279246.js
+++ b/netwerk/test/unit/test_bug1279246.js
@@ -91,6 +91,6 @@ function handler(metadata, response) {
   bos.setOutputStream(response.bodyOutputStream);
 
   response.processAsync();
-  bos.writeByteArray(responseBody, responseBody.length);
+  bos.writeByteArray(responseBody);
   response.finish();
 }
diff --git a/netwerk/test/unit/test_content_encoding_gzip.js b/netwerk/test/unit/test_content_encoding_gzip.js
index cb4412ef817d..60e5838d6a4b 100644
--- a/netwerk/test/unit/test_content_encoding_gzip.js
+++ b/netwerk/test/unit/test_content_encoding_gzip.js
@@ -203,6 +203,6 @@ function handler(metadata, response) {
   bos.setOutputStream(response.bodyOutputStream);
 
   response.processAsync();
-  bos.writeByteArray(tests[index].body, tests[index].body.length);
+  bos.writeByteArray(tests[index].body);
   response.finish();
 }
diff --git a/netwerk/test/unit/test_gzipped_206.js b/netwerk/test/unit/test_gzipped_206.js
index a0f56a36e87c..0f7e2d266ee7 100644
--- a/netwerk/test/unit/test_gzipped_206.js
+++ b/netwerk/test/unit/test_gzipped_206.js
@@ -118,7 +118,7 @@ function cachedHandler(metadata, response) {
   bos.setOutputStream(response.bodyOutputStream);
 
   response.processAsync();
-  bos.writeByteArray(body, body.length);
+  bos.writeByteArray(body);
   response.finish();
 }
 
diff --git a/netwerk/test/unit/test_range_requests.js b/netwerk/test/unit/test_range_requests.js
index 8a6d9f9c7bef..6d8e36bc5944 100644
--- a/netwerk/test/unit/test_range_requests.js
+++ b/netwerk/test/unit/test_range_requests.js
@@ -261,7 +261,7 @@ function handler_4(metadata, response) {
       );
       bos.setOutputStream(response.bodyOutputStream);
       response.processAsync();
-      bos.writeByteArray(body, body.length);
+      bos.writeByteArray(body);
       response.finish();
       break;
     case 1:
diff --git a/netwerk/test/unit/test_reply_without_content_type.js b/netwerk/test/unit/test_reply_without_content_type.js
index 4a99389e2819..2c48c16a8c89 100644
--- a/netwerk/test/unit/test_reply_without_content_type.js
+++ b/netwerk/test/unit/test_reply_without_content_type.js
@@ -129,7 +129,7 @@ function serverHandler_GZip(metadata, response) {
     Ci.nsIBinaryOutputStream
   );
   bos.setOutputStream(response.bodyOutputStream);
-  bos.writeByteArray(httpbodyGZip, httpbodyGZip.length);
+  bos.writeByteArray(httpbodyGZip);
   if (dbg) {
     print("============== serverHandler GZip: out");
   }
diff --git a/security/manager/ssl/nsNSSCertificate.cpp b/security/manager/ssl/nsNSSCertificate.cpp
index f09f456abe48..a82b78fe3591 100644
--- a/security/manager/ssl/nsNSSCertificate.cpp
+++ b/security/manager/ssl/nsNSSCertificate.cpp
@@ -13,6 +13,7 @@
 #include "mozilla/Base64.h"
 #include "mozilla/Casting.h"
 #include "mozilla/NotNull.h"
+#include "mozilla/Span.h"
 #include "mozilla/Unused.h"
 #include "nsArray.h"
 #include "nsCOMPtr.h"
@@ -1194,7 +1195,7 @@ nsNSSCertificate::Write(nsIObjectOutputStream* aStream) {
   if (NS_FAILED(rv)) {
     return rv;
   }
-  return aStream->WriteByteArray(mCert->derCert.data, mCert->derCert.len);
+  return aStream->WriteBytes(AsBytes(MakeSpan(mCert->derCert.data, mCert->derCert.len)));
 }
 
 NS_IMETHODIMP
diff --git a/toolkit/components/downloads/test/unit/common_test_Download.js b/toolkit/components/downloads/test/unit/common_test_Download.js
index e88d571da988..6227a0abaf31 100644
--- a/toolkit/components/downloads/test/unit/common_test_Download.js
+++ b/toolkit/components/downloads/test/unit/common_test_Download.js
@@ -1809,10 +1809,7 @@ add_task(async function test_with_content_encoding() {
     );
 
     let bos = new BinaryOutputStream(aResponse.bodyOutputStream);
-    bos.writeByteArray(
-      TEST_DATA_SHORT_GZIP_ENCODED,
-      TEST_DATA_SHORT_GZIP_ENCODED.length
-    );
+    bos.writeByteArray(TEST_DATA_SHORT_GZIP_ENCODED);
   });
 
   let download = await promiseStartDownload(sourceUrl);
@@ -1849,10 +1846,7 @@ add_task(async function test_with_content_encoding_ignore_extension() {
     );
 
     let bos = new BinaryOutputStream(aResponse.bodyOutputStream);
-    bos.writeByteArray(
-      TEST_DATA_SHORT_GZIP_ENCODED,
-      TEST_DATA_SHORT_GZIP_ENCODED.length
-    );
+    bos.writeByteArray(TEST_DATA_SHORT_GZIP_ENCODED);
   });
 
   let download = await promiseStartDownload(sourceUrl);
diff --git a/toolkit/components/downloads/test/unit/head.js b/toolkit/components/downloads/test/unit/head.js
index 42cdbb7549df..7cbc9b669283 100644
--- a/toolkit/components/downloads/test/unit/head.js
+++ b/toolkit/components/downloads/test/unit/head.js
@@ -870,17 +870,11 @@ add_task(function test_common_initialize() {
       );
 
       let bos = new BinaryOutputStream(aResponse.bodyOutputStream);
-      bos.writeByteArray(
-        TEST_DATA_SHORT_GZIP_ENCODED_FIRST,
-        TEST_DATA_SHORT_GZIP_ENCODED_FIRST.length
-      );
+      bos.writeByteArray(TEST_DATA_SHORT_GZIP_ENCODED_FIRST);
     },
     function secondPart(aRequest, aResponse) {
       let bos = new BinaryOutputStream(aResponse.bodyOutputStream);
-      bos.writeByteArray(
-        TEST_DATA_SHORT_GZIP_ENCODED_SECOND,
-        TEST_DATA_SHORT_GZIP_ENCODED_SECOND.length
-      );
+      bos.writeByteArray(TEST_DATA_SHORT_GZIP_ENCODED_SECOND);
     }
   );
 
diff --git a/toolkit/components/mediasniffer/test/unit/test_mediasniffer_ext.js b/toolkit/components/mediasniffer/test/unit/test_mediasniffer_ext.js
index ce22947be611..1083c427dee8 100644
--- a/toolkit/components/mediasniffer/test/unit/test_mediasniffer_ext.js
+++ b/toolkit/components/mediasniffer/test/unit/test_mediasniffer_ext.js
@@ -111,7 +111,7 @@ function handler(metadata, response) {
   response.setHeader("Content-Type", "", false);
   var body = getFileContents(do_get_file(tests[testRan].path));
   var bos = new BinaryOutputStream(response.bodyOutputStream);
-  bos.writeByteArray(body, body.length);
+  bos.writeByteArray(body);
 }
 
 function run_test() {
diff --git a/toolkit/components/places/PageIconProtocolHandler.jsm b/toolkit/components/places/PageIconProtocolHandler.jsm
index 9758a47fcf10..63bf446c51d0 100644
--- a/toolkit/components/places/PageIconProtocolHandler.jsm
+++ b/toolkit/components/places/PageIconProtocolHandler.jsm
@@ -49,13 +49,13 @@ function streamDefaultFavicon(uri, loadInfo, outputStream, originalChannel) {
   }
 }
 
-function serveIcon(pipe, data, len) {
+function serveIcon(pipe, data) {
   // Pass the icon data to the output stream.
   let stream = Cc["@mozilla.org/binaryoutputstream;1"].createInstance(
     Ci.nsIBinaryOutputStream
   );
   stream.setOutputStream(pipe.outputStream);
-  stream.writeByteArray(data, len);
+  stream.writeByteArray(data);
   stream.close();
   pipe.outputStream.close();
 }
@@ -109,7 +109,10 @@ PageIconProtocolHandler.prototype = {
             try {
               channel.contentType = mimeType;
               channel.contentLength = len;
-              serveIcon(pipe, data, len);
+              if (len != data.length) {
+                throw new Error("Unexpected data length");
+              }
+              serveIcon(pipe, data);
             } catch (ex) {
               streamDefaultFavicon(uri, loadInfo, pipe.outputStream, channel);
             }
diff --git a/toolkit/components/telemetry/tests/unit/test_TelemetryLateWrites.js b/toolkit/components/telemetry/tests/unit/test_TelemetryLateWrites.js
index 22a0f75c5375..4dcae48ea138 100644
--- a/toolkit/components/telemetry/tests/unit/test_TelemetryLateWrites.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryLateWrites.js
@@ -48,7 +48,7 @@ function write_string_to_file(file, contents) {
   bos.setOutputStream(ostream);
 
   let utf8 = new TextEncoder("utf-8").encode(contents);
-  bos.writeByteArray(utf8, utf8.length);
+  bos.writeByteArray(utf8);
   ostream.QueryInterface(Ci.nsISafeOutputStream).finish();
   ostream.close();
 }
diff --git a/xpcom/io/nsBinaryStream.cpp b/xpcom/io/nsBinaryStream.cpp
index 2bf7cb36a93d..3995bda228b1 100644
--- a/xpcom/io/nsBinaryStream.cpp
+++ b/xpcom/io/nsBinaryStream.cpp
@@ -26,6 +26,7 @@
 #include "mozilla/EndianUtils.h"
 #include "mozilla/PodOperations.h"
 #include "mozilla/RefPtr.h"
+#include "mozilla/Span.h"
 #include "mozilla/UniquePtr.h"
 
 #include "nsCRT.h"
@@ -41,8 +42,11 @@
 #include "js/RootingAPI.h"  // JS::{Handle,Rooted}
 #include "js/Value.h"       // JS::Value
 
+using mozilla::AsBytes;
+using mozilla::MakeSpan;
 using mozilla::MakeUnique;
 using mozilla::PodCopy;
+using mozilla::Span;
 using mozilla::UniquePtr;
 
 already_AddRefed<nsIObjectOutputStream> NS_NewObjectOutputStream(
@@ -206,11 +210,8 @@ nsBinaryOutputStream::WriteStringZ(const char* aString) {
 
 NS_IMETHODIMP
 nsBinaryOutputStream::WriteWStringZ(const char16_t* aString) {
-  uint32_t length, byteCount;
-  nsresult rv;
-
-  length = NS_strlen(aString);
-  rv = Write32(length);
+  uint32_t length = NS_strlen(aString);
+  nsresult rv = Write32(length);
   if (NS_FAILED(rv)) {
     return rv;
   }
@@ -218,10 +219,9 @@ nsBinaryOutputStream::WriteWStringZ(const char16_t* aString) {
   if (length == 0) {
     return NS_OK;
   }
-  byteCount = length * sizeof(char16_t);
 
 #ifdef IS_BIG_ENDIAN
-  rv = WriteBytes(reinterpret_cast<const char*>(aString), byteCount);
+  rv = WriteBytes(AsBytes(MakeSpan(aString, length)));
 #else
   // XXX use WriteSegments here to avoid copy!
   char16_t* copy;
@@ -229,14 +229,14 @@ nsBinaryOutputStream::WriteWStringZ(const char16_t* aString) {
   if (length <= 64) {
     copy = temp;
   } else {
-    copy = reinterpret_cast<char16_t*>(malloc(byteCount));
+    copy = static_cast<char16_t*>(malloc(length * sizeof(char16_t)));
     if (!copy) {
       return NS_ERROR_OUT_OF_MEMORY;
     }
   }
   NS_ASSERTION((uintptr_t(aString) & 0x1) == 0, "aString not properly aligned");
   mozilla::NativeEndian::copyAndSwapToBigEndian(copy, aString, length);
-  rv = WriteBytes(reinterpret_cast<const char*>(copy), byteCount);
+  rv = WriteBytes(AsBytes(MakeSpan(copy, length)));
   if (copy != temp) {
     free(copy);
   }
@@ -250,24 +250,29 @@ nsBinaryOutputStream::WriteUtf8Z(const char16_t* aString) {
   return WriteStringZ(NS_ConvertUTF16toUTF8(aString).get());
 }
 
-NS_IMETHODIMP
-nsBinaryOutputStream::WriteBytes(const char* aString, uint32_t aLength) {
+nsresult nsBinaryOutputStream::WriteBytes(Span<const uint8_t> aBytes) {
   nsresult rv;
   uint32_t bytesWritten;
 
-  rv = Write(aString, aLength, &bytesWritten);
+  rv = Write(reinterpret_cast<const char*>(aBytes.Elements()), aBytes.Length(),
+             &bytesWritten);
   if (NS_FAILED(rv)) {
     return rv;
   }
-  if (bytesWritten != aLength) {
+  if (bytesWritten != aBytes.Length()) {
     return NS_ERROR_FAILURE;
   }
   return rv;
 }
 
 NS_IMETHODIMP
-nsBinaryOutputStream::WriteByteArray(uint8_t* aBytes, uint32_t aLength) {
-  return WriteBytes(reinterpret_cast<char*>(aBytes), aLength);
+nsBinaryOutputStream::WriteBytesFromJS(const char* aString, uint32_t aLength) {
+  return WriteBytes(AsBytes(MakeSpan(aString, aLength)));
+}
+
+NS_IMETHODIMP
+nsBinaryOutputStream::WriteByteArray(const nsTArray<uint8_t>& aByteArray) {
+  return WriteBytes(aByteArray);
 }
 
 NS_IMETHODIMP
@@ -340,7 +345,7 @@ nsBinaryOutputStream::WriteID(const nsIID& aIID) {
     return rv;
   }
 
-  rv = WriteBytes(reinterpret_cast<const char*>(&aIID.m3[0]), sizeof(aIID.m3));
+  rv = WriteBytes(aIID.m3);
   if (NS_WARN_IF(NS_FAILED(rv))) {
     return rv;
   }
diff --git a/xpcom/io/nsIBinaryOutputStream.idl b/xpcom/io/nsIBinaryOutputStream.idl
index 4081f51fb365..ae9c5062f1ef 100644
--- a/xpcom/io/nsIBinaryOutputStream.idl
+++ b/xpcom/io/nsIBinaryOutputStream.idl
@@ -5,6 +5,13 @@
 
 #include "nsIOutputStream.idl"
 
+%{C++
+namespace mozilla {
+template<class ElementType, size_t Extent> class Span;
+}
+%}
+native Bytes(mozilla::Span<const uint8_t>);
+
 /**
  * This interface allows writing of primitive data types (integers,
  * floating-point values, booleans, etc.) to a stream in a binary, untagged,
@@ -55,15 +62,20 @@ interface nsIBinaryOutputStream : nsIOutputStream {
     /**
      * Write an opaque byte array to the stream.
      */
+    [binaryname(WriteBytesFromJS)]
     void writeBytes([size_is(aLength)] in string aString,
                     [optional] in uint32_t aLength);
 
+    /**
+     * Non-scriptable and saner-signature version of the same.
+     */
+    [noscript, nostdcall, binaryname(WriteBytes)]
+    void writeBytesNative(in Bytes aBytes);
+
     /**
      * Write an opaque byte array to the stream.
      */
-    void writeByteArray([array, size_is(aLength)] in uint8_t aBytes,
-                        [optional] in uint32_t aLength);
-
+    void writeByteArray(in Array<uint8_t> aBytes);
 };
 
 %{C++
diff --git a/xpcom/tests/unit/test_storagestream.js b/xpcom/tests/unit/test_storagestream.js
index 95e0c7f4fc45..2a0fb1b5696d 100644
--- a/xpcom/tests/unit/test_storagestream.js
+++ b/xpcom/tests/unit/test_storagestream.js
@@ -117,7 +117,7 @@ function test4() {
   );
   bos.setOutputStream(outStream);
 
-  bos.writeByteArray(bytes, bytes.length);
+  bos.writeByteArray(bytes);
   bos.close();
 
   var inp = ss.newInputStream(0);
diff --git a/xpcom/tests/unit/test_streams.js b/xpcom/tests/unit/test_streams.js
index 6ec1c8dd6323..212b41cc8ebc 100644
--- a/xpcom/tests/unit/test_streams.js
+++ b/xpcom/tests/unit/test_streams.js
@@ -67,13 +67,13 @@ function test_binary_streams() {
   Assert.equal(msg, HelloStr);
   msg = null;
   countObj.value = -1;
-  os.writeByteArray(HelloArray, HelloArray.length);
+  os.writeByteArray(HelloArray);
   Assert.equal(is.available(), HelloStr.length);
   msg = is.readByteArray(HelloStr.length);
   Assert.equal(typeof msg, typeof HelloArray);
   Assert.equal(msg.toSource(), HelloArray.toSource());
   Assert.equal(is.available(), 0);
-  os.writeByteArray(HelloArray, HelloArray.length);
+  os.writeByteArray(HelloArray);
   Assert.equal(
     is.readArrayBuffer(buffer.byteLength, buffer),
     HelloArray.length
@@ -96,7 +96,7 @@ function test_binary_streams() {
   os.writeStringZ("Mozilla");
   os.writeWStringZ("Gecko");
   os.writeBytes(HelloStr, HelloStr.length);
-  os.writeByteArray(HelloArray, HelloArray.length);
+  os.writeByteArray(HelloArray);
   // Available should not be zero after a long write like this.
   Assert.notEqual(is.available(), 0);