I've just updated my CF Debug Copy for Firefox extension to work with the Firefox 3 Beta. To install just click the link below.
I've labeled this version 1.1b1. Once Firefox 3 is released I'll remove the 'b1'. I also added the source to RIAForge SVN if anyone is interested.
(Note: Although I haven't tested on Firefox 1.5, this version of the extension should work with any Firefox version between 1.5 and 3.)