diff --git a/uriloader/exthandler/tests/mochitest/Makefile.in b/uriloader/exthandler/tests/mochitest/Makefile.in
index a96c2668ff2..9bea6f6297b 100644
--- a/uriloader/exthandler/tests/mochitest/Makefile.in
+++ b/uriloader/exthandler/tests/mochitest/Makefile.in
@@ -16,6 +16,7 @@ MOCHITEST_FILES =	\
 		handlerApps.js \
 		handlerApp.xhtml \
 		unsafeBidiFileName.sjs \
+		test_badMimeType.html \
 		$(NULL)
 
 include $(topsrcdir)/config/rules.mk
diff --git a/uriloader/exthandler/tests/mochitest/test_badMimeType.html b/uriloader/exthandler/tests/mochitest/test_badMimeType.html
new file mode 100644
index 00000000000..1d85f97bdc7
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/test_badMimeType.html
@@ -0,0 +1,27 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <title>Crashtest for bad MIME type</title>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<pre id="test">
+<script class="testbody" type="text/javascript">
+  const Cc = SpecialPowers.Cc;
+  const Ci = SpecialPowers.Ci;
+  const mimeService = Cc["@mozilla.org/mime;1"].
+                      getService(Ci.nsIMIMEService);
+  // "text/plain" has an 0xFF character appended to it.  This means it's an
+  // invalid string, which is tricky to enter using a text editor (I used
+  // emacs' hexl-mode).  It also means an ordinary text editor might drop it
+  // or convert it to something that *is* valid (in UTF8).  So we measure
+  // its length to make sure this hasn't happened.
+  var badMimeType = "text/plainÿ";
+  ok(badMimeType.length == 11, "badMimeType has changed, making this test invalid");
+  mimeService.getFromTypeAndExtension(badMimeType, "txt");
+  ok(true, "The test shouldn't trigger a crash");
+</script>
+</pre>
+</body>
+</html>
