This is a test case for jQuery pull request 128: jquery/jquery#128 To run the code, create an extension proxy file as described here: https://developer.mozilla.org/en/setting_up_extension_development_environment 1. Run Firefox 4 with this extension installed 2. Open any webpage 3. Open the error console Expected: no exception in jquery-1.4.4 Actual: exception in jquery-1.4.4.js on line 1091