Not denying that what is "under the hood" may be an improvement but,foe me,many of the other cosmetic changes are totally unnecessary - I still prefer my menu bar to the new Firefox Button - I still like my status bar (now called add-on bar! - why!)along the bottom and I prefer my page loading progress bar on the status bar not along the browser search bar. I'm not against progress but when your hand and eye move around the screen automatically ,and have done for years,the retraining process has to offer a bit more than the changes here to justify the effort.
Graham Even though I installed mine into a different folder rather than let it over write the existing version they both write to the same profile in application data so you are as well to take a copy of your existing profile first. You can then reinstate your current profile if you wish to go back to 3.6.15 and it will be uncorrupted in any way by the new installation.
Norton tool bar which incorporates Identity Safe for auto logins, and Met Office gadget will not work with FF4 at the moment. Don't know about any others so I will give it a miss until that gets sorted out. Both these worked right away with the new IE9.