Theres a page on mozilla.org which highlights most of the known issues with add-ons.
Worth perusing before downloading any add-ons imho.Hopefully 3.5 fixes most of them
http://kb.mozillazine.org/Problematic_extensions
Maybe a bit dated but this list is far more user friendly than the search engine on the mozilla.org add-on page
http://www.econsultant.com/i-want-firefox-extension/
KeyScrambler Personal encrypts your keystrokes at the kernel driver level to protect your information from keyloggers.
https://addons.mozilla.org/en-US/firefox/addon/3383
Various versions of Error Zilla which gives you a legible error message rather than the usual
technical speak that you usually get
https://addons.mozilla.org/en-US/fir...rzilla&cat=all