How has noone suggested Classic Theme Restorer for Firefox! This is what allows you to restore the pre-Australis theme that (imho) marked one of the more serious nails in the coffin for Firefox.
I also use:
- FireGestures, for mouse gestures (how people work without right-click-scroll-up/down to go to the top and bottom of pages is beyond me).
- FoxyProxy, so I can use different proxies for specific URLs, mostly allowing me to test geoip stuff but also coincidentally allowing me to avoid geographical restrictions.
- Flashblock, although I've just realised I can remove this as I've uninstalled Flash months ago so it is kind of redundant.
- Modify Headers, for development so you can easily inspect & modify HTTP headers.
- Tamper Data, as above.