Possible Duplicate:
Do disabled Firefox extensions make Firefox slower?
In case 'it depends on the type of extension': Ones in my disabled list include DownThemAll, DownloadHelper, DivX Plus web player, Adobe Acrobat create pdf, and Adobe Contribute toolbar.
Disabling extensions is enough - a disabled extension is as good as not present. There is still a small startup delay due to that extension being in the database but it is absolutely negligible (I tried measuring it once but it is way below the usual startup time variance on my system).
