For anyone new to Firefox (or those thinking about upgrading to 0.9) i would recommend waiting until 0.91 comes out, which is supposed to fix a number of rather nasty bugs. Last i heard it should be coming within a few days. For those that can't wait, be careful when installing extensions for 0.9. I suggest reading through the user comments section that accompanies each extension on the download page before you install an extension so that you'll know if other users have experienced any trouble with it. Also when upgrading an extension with 0.9 you should uninstall the old extension before you install the newer one.
Hope none of this scares anyone away from trying it. I absolutely love Firefox and Thunderbird (especially extensions like All-in-One Gestures, DictionarySearch, and Adblock) but they are still in the testing stages and so problems can occasionally crop up.