It isn’t quite out yet, but Matthew D. Sarrel at PC Magazine reviews Release Candidate 3 of Firefox:
The best browser gets even better with improved tab controls, reworked user preferences, a more robust extensions system, faster page loading and better security.
He gives it 4.5 out of 5 The detailed review is here and while there are some UI changes, the big changes are mostly behind the scenes. The only downside I see:
Firefox 1.5 includes an entirely new extension system, which unfortunately means that many older extensions won’t work (Adblock and Forecast Fox worked properly on our test machine). Developers are updating their extensions as you read this, so the updated extensions could be in pretty good shape by the time 1.5 launches. Mozilla has even revamped the extensions-installation system. Although the process worked well in most instances, the installation, upgrade, and uninstall subsystems were prone to error. When it was installing a new version of an extension, it occasionally left files from older versions behind, which could lead to incompatibilities and browser instability.
I guess it’s all to the good, but it still means taking an extension inventory and checking the compatibility or availabilty of new versions before you upgrade.