diff --git a/parser/htmlparser/nsExpatDriver.cpp b/parser/htmlparser/nsExpatDriver.cpp
index f5804c70463e..8a23449e63ec 100644
--- a/parser/htmlparser/nsExpatDriver.cpp
+++ b/parser/htmlparser/nsExpatDriver.cpp
@@ -828,6 +828,10 @@ nsresult nsExpatDriver::HandleError() {
                                                 nsPrintfCString("%u", code)},
             mozilla::Telemetry::EventExtraEntry{
                 "location"_ns, nsPrintfCString("%u:%u", lineNumber, colNumber)},
+            mozilla::Telemetry::EventExtraEntry{
+                "last_line"_ns, NS_ConvertUTF16toUTF8(mLastLine)},
+            mozilla::Telemetry::EventExtraEntry{
+                "last_line_len"_ns, nsPrintfCString("%u", mLastLine.Length())},
         });
 
     mozilla::Telemetry::SetEventRecordingEnabled("ysod"_ns, true);
diff --git a/parser/htmlparser/tests/mochitest/browser_ysod_telemetry.js b/parser/htmlparser/tests/mochitest/browser_ysod_telemetry.js
index 331e453afc8b..5a709351e70a 100644
--- a/parser/htmlparser/tests/mochitest/browser_ysod_telemetry.js
+++ b/parser/htmlparser/tests/mochitest/browser_ysod_telemetry.js
@@ -30,7 +30,12 @@ add_task(async function test_popup_opened() {
         method: "shown",
         object: "ysod",
         value: PAGE_URL.substr(0, 80),
-        extra: { error_code: "11", location: "3:12" },
+        extra: {
+          error_code: "11",
+          location: "3:12",
+          last_line: "    <title>&nonExistingEntity;</title>",
+          last_line_len: "38",
+        },
       },
     ],
     {
diff --git a/toolkit/components/telemetry/Events.yaml b/toolkit/components/telemetry/Events.yaml
index 7e6043dfea83..f1c3a7f705c6 100644
--- a/toolkit/components/telemetry/Events.yaml
+++ b/toolkit/components/telemetry/Events.yaml
@@ -2567,3 +2567,5 @@ ysod:
     extra_keys:
       error_code: Code of the XML Error (See `XML_Error` enum in expat.h for the list of errors where XML_ERROR_NONE is 0).
       location: Location as Row:Column of where the error happened.
+      last_line: The content of the last line that led to the error.
+      last_line_len: The length of the last line that led to the error.