Today I upgraded Firefox to 2.0.7, which fixes a flaw in the QuickTime plugin. However, after the upgrade Firefox didn't load any of the extensions I have installed
Fortunately I found this thread in the support forum. Simply delete extensions.ini, extensions.cache, and extensions.rdf from your profile folder. On its next start Firefox will scan for installed extensions and regenerate these files.
I hope this post will be indexed properly so you can find this information faster than me.