This add-on will let the arrow show history the way it used to.
It’s crashing on me once every few minutes, unexpectedly, and no matter what site I’m on. 
I tried v4 when it first got released, but went back after some of my extensions and a few other weird things were getting on my nerves (something was causing visited links on the SDMB to not change colour). So I’m going to stick with 3.6 for a while longer, until versions 5 and even 6 come out, because being an early adopter would just frustrate me.
All in all, I’m satisfied with FF 4, but I was hoping that it finally had a decent memory management, but unfortunately it’s still a resource hog if you’re working with lots of tabs like I use to do. The UI changes are ok, and it’s no big deal to tweak it to your preferred style.
As for blurry fonts: I had the same problem first, but this tweak worked for me.
ETA: Had no problems with NoScript and AdBlock, as well as with all my other add-ons.
This is just a silly thing, and it might just be me being stupid, but the right-click on a link options have changed. The top option used to be “in a new window” and the second “in a new tab”. This is now reversed, and my muscle memory (really, just done it so often without looking) leads me to open new windows all the time. A minor bugbear, for sure. Otherwise, it is noticeably faster than Firefox 3.x
A tip: Clicking with the mouse wheel also opens the link in a new tab.
I updated it both on my home and work PCs.
At home I had to spend an hour or more to get it to work in any acceptable fashion. Load times for web pages were horrific under the default install: 60+ seconds for google.com, 500-600 seconds for straightdope.com.
At work I never could get it working for more than 10-15 minutes at a time without crashes. And on that PC, I finally went and downgraded it back to 3.6.
Stylistically, I don’t like the layout changes. The default settings (hardware acceleration for one) seem to pretty poor for many (if not most) users. Add that to the technical problems I encountered on both of the PCs I tried it on, and I wouldn’t recommend it at this point.
No “organise bookmarks” option in the Bookmarks menu.