diff --git a/dom/system/IOUtils.cpp b/dom/system/IOUtils.cpp
index 2314a008b954..26183864f698 100644
--- a/dom/system/IOUtils.cpp
+++ b/dom/system/IOUtils.cpp
@@ -932,7 +932,7 @@ Result<uint32_t, IOUtils::IOError> IOUtils::WriteSync(
     nsCOMPtr<nsIFile> toMove;
     MOZ_ALWAYS_SUCCEEDS(aFile->Clone(getter_AddRefs(toMove)));
 
-    bool noOverwrite = aOptions.mMode != WriteMode::Create;
+    bool noOverwrite = aOptions.mMode == WriteMode::Create;
 
     if (MoveSync(toMove, backupFile, noOverwrite).isErr()) {
       return Err(IOError(NS_ERROR_FILE_COPY_OR_MOVE_FAILED)
diff --git a/dom/system/tests/ioutils/test_ioutils_read_write.html b/dom/system/tests/ioutils/test_ioutils_read_write.html
index 5b72a7b655ef..6726e8e25b5e 100644
--- a/dom/system/tests/ioutils/test_ioutils_read_write.html
+++ b/dom/system/tests/ioutils/test_ioutils_read_write.html
@@ -153,6 +153,7 @@
 
       info("Test backup with tmp and backup file options, existing destination");
       let newFileContents = new TextEncoder().encode("New file contents");
+      ok(await fileExists(destFileName), `Expected ${destFileName} to exist`);
       bytesWritten =
         await IOUtils.write(destFileName, newFileContents, {
           backupFile: backupFileName,
@@ -174,6 +175,31 @@
         "IOUtils::write IOUtils::write can move tmp file to destination after performing a backup"
       );
 
+      info("Test backup with tmp and backup file options, existing destination and backup");
+      newFileContents = new TextEncoder().encode("Updated new file contents");
+      ok(await fileExists(destFileName), `Expected ${destFileName} to exist`);
+      ok(await fileExists(backupFileName), `Expected ${backupFileName} to exist`);
+      bytesWritten =
+        await IOUtils.write(destFileName, newFileContents, {
+          backupFile: backupFileName,
+          tmpPath: tmpFileName,
+        });
+
+      ok(!await fileExists(tmpFileName), "IOUtils::write cleans up the tmpFile");
+      ok(
+        await fileHasTextContents(backupFileName, "New file contents"),
+        "IOUtils::write can create a backup if the target file exists"
+      );
+      ok(
+        await fileHasTextContents(destFileName, "Updated new file contents"),
+        "IOUtils::write can write to the destination when a temporary file is used"
+      );
+      is(
+        bytesWritten,
+        newFileContents.length,
+        "IOUtils::write IOUtils::write can move tmp file to destination after performing a backup"
+      );
+
       await cleanup(destFileName, backupFileName);
     });