I have upgraded my Firefox pre-release 1.0 to Firefox 1.0. Guess what, some of the extensions I have been dependent upon disappeared, sort of.
The first is the spelling checker. I was shocked after I wrote this piece and right click. No more spelling check! Luckily, it was easily resolved by going to http://spellbound.sourceforge.net/install.htm and do a reinstallation.
The spiderzilla, a website download tool, is gone as well. Again, a reinstallation from http://spiderzilla.mozdev.org/ solved the problem.
Sage, RSS reader and aggregation is also gone. ditto at http://sage.mozdev.org/.
So, when you upgrade your Firefox, be prepared for some work to get the extensions you have become used to.