I just installed Firefox 0.9, then 5 minutes later switched back to 0.8.
Upgrading lost all my browser extensions, and it seems that the extensions need to be modifed to work with 0.9 so there was a good chance none of them would work.
Sod that.
I’ll try again in a few weeks.
I’ll wait a while too, until the plugins that I use are available for 0.9; the Google toolbar, the Tabbrowser extensions, IE View and Linky.