I just downloaded Firefox 3, it was a fast download (about 7Mb), double clicked the installer and the program was installed very quickly but then i had my first problem, I couldn't see all the installed add-ons because the list wouldn't let me scroll down to the last item, so I just continued with the configuration, then I got a message telling me a lot of my add-ons and themes where not compatible with version 3, so I just continued to test the program but found out that Tab Mix Plus wasn't supported nor my Vista Aero theme (my favorites), I was so used to all my add-ons that I decided to go back to Firefox 2, so I just installed ver 2 without unistalling ver 3 before (I did it that way because my previous experience with Firefox installations didn't gave me any problems) so when I tried to start ver 2 I got an exception and Firefox wouldn't start, did a reboot and the exception continued and FF wouldn't start, so i decided to uninstall it and reinstall ver 2 again and after that everything came back to normal : ) wich is great and it's one of the things I love from Firefox.
Now, I wouldn't recommend to install Firefox 3 if you love your add-ons because a lot of them won't work, in my case im gonna wait till all my favorite extensions migrate to ver 3, what frustrates me is that mozilla webpage should explained this add-on situation to me before so I din't waste my time, and maybe Mozilla should have exhorted add-ons programmers to migrate to ver 3 before they launched a new Firerox release, i mean ver 3 betas have been out there for months. Firefox is expecting to break a download guiness record on their new release 3 but wouln't it be better if all the people that downloads the program could use their favorite add-ons and themes? and continue using ver 3?
Of course if you are a new user it is a great moment to start using Firefox 3 specially if you've been using IE.
Im sure ver 3 is amazing and i'm anxious to give it a try, but not for the moment.
Update: All my add-ons where updated pretty fast to ver 3, FF3 Rocks.