Yesterday Firefox went crazy on me. Attempting to add a bookmark in any of the four ways crashed the browser, not just randomly, but every frigging time.
I thought I'd run it from the command line and see if any console messages were thrown. Big mistake; turned out that v1.5 had precedence in the path, and it grabbed this opportunity with both hands. It first informed me that Firefox was not my default browser, and asked me whether I wanted to make it so. It then proceeded to trash my extensions and themes gleefully: none of them worked when I opened 2.0 again.
I left things in this state last night. Before embarking on a painful download today morning, I wanted to check whether I could tweak the profile files and fix things. I renamed the extensions.cache and extensions.rdf files, and voila, the add-ons returned as if nothing had happened, and even had the gall to pretend to ignore me when I asked them where the fsck they had been.