1) Since 1.5 was release some of my extension have stopped working and there are no upgrades for 1.5 (e.g. spellChecker extension). To adress this use the following trick (and be aware that it might make your firefox less stable, though I have no problem on this matter, yet at least)

  1. At the location bar, enter: about:config. This will show you a list of Firefox internal preferences.
  2. Right-click on the list, select New > String
  3. Enter “app.extensions.version” (without quotes) for the preference name.
  4. Then, enter “1.0″ (without quotes) as the value for app.extensions.version.
  5. Restart Firefox 1.5, then enable those disabled Firefox extensions.
  6. Restart Firefox 1.5 again to active the extensions. Done.

2) Chrome Cleaner Extension - Very Likely to help speed up your firefox (both mac and pc)

3) And here is and excellent list of extensions for your pleasure.