So while we didn't have anything on Firefox 4's numbers, we had more downloads in 30 hours than we did in a month on any of the individual betas. Not bad!
The change that I landed last-minute due to the crash that cropped up does have a performance impact, but some users are seeing a hit significantly worse than my measurements. I'll talk about that in a minute. If you are one of those users, and you weren't crashing in b12, you could revert to that temporarily pending followup. Note that you will get pestered to update unless you turn that off too.
I should also use this spot to state, since it was a big concern to people in the previous entry, that plugins will have glitches. The code holding it together is kludgy at best and will eventually not work, because these plugins are no longer updated for PPC, and I don't know what the changes occurring in "Firefox 5" will do either. Right now most of the glitches are limited to scrolling artifacts. I have always suggested installing an add-on such as Flashblock, which is what I do myself, and then you can enable only the Flash applets you actually need to see. Remember, plugins will be pref'ed off by default in the next major release due to imminent security and compatibility issues.
Because both of these changes are a little risky, there will be a single minor beta (I think there's a Jonathan Coulton song by that title :P ) incorporating both new updates. I hope to have that out to play with in a couple of weeks. You will not be automatically updated to it -- watch this space for more.