Get [[wp:Greasemonkey]] ([https://addons.mozilla.org/en-US/firefox/addon/748 here]) so that you can have your browser be even more useful. You might even want to use [[http://monkey.coolspacer.com/ Super Greasemonkey]] if you like using the awesome [http://jquery.com/ JQuery] library. I'm hoping that Super Greasemonkey makes it into the official version, and that you can specify in your script which JQuery library to run (like Google's hosted JQuery scripts) making it easier to keep compatibility with the JQuery API version and also avoiding the need to bundle JQuery.
* [[MediaWiki/js]] talks about using JavaScript in MediaWiki