Most of the Firefox 4 bugs have been resolved in the extensive testing period before the release. However, there are some add-ons that are not yet compatible with Firefox 4. Most add-ons are not developed by Mozilla and, therefore, Mozilla has no control over their updates. The major issue with upgrading to Firefox 4, as with any upgrade, is incompatible add-ons. If you have any important add-ons that you cannot use Firefox without, you should check to see if they have been updated to work with Firefox 4 yet. If you do have any other issues, you can always come back here to resolve them.

Many of the questions here have not been about bugs. There have been many questions about changes to how Firefox works, new features, changes to the appearance and theme, incompatible add-ons, and it no longer supporting OS X 10.4 and PPC Macs. Most questions have not been about bugs.

I have found Firefox 4 to be very stable and faster than previous releases.

how do you figure whether the addons one uses is compatible with 4.0? Thanks

flagreg, the following link gives the compatibility info for many add-ons. It does not include every add-on but does include all of the popular add-ons hosted on the Mozilla add-ons site. Those listed as 4.0.* or higher should work on Firefox 4

https://addons.mozilla.org/en-US/firefox/compatibility/report