Almost forgot, LOL

1. JSView: Debugging and source code stuff...
2. Total Validator: as the mane suggests...
3. Firebug: Debugging and source code stuff...
4. GMail Manager: pretty neat...
5. WebDeveloper toolbar: YOU MUST HAVE THIS
6. IE Tab: Let's you see pages in FF using the IE engine. It's IE and Firefox in one.
7. SEOpen: neat SEO tools

and a few other beauties.... check the plugins page:
https://addons.mozilla.org/