I’ve been using and promoting Firefox since 
the beginning when it was named Phoenix back in 2002. The browser has improved steadily in 8-1/2 years getting better with every release. The availability of version 4.0 this week brings major improvements in speed and usability, so naturally I had to upgrade. Since I didn’t participate in the beta testing and I also didn’t read up enough on this version, my version 4 experience was not what I’d anticipated.
This brings up my first tip, update your video driver or check the MozillaWiki to make sure your driver version is OK. I had very old Nvidia drivers so Firefox had to turn off the some of its new features. This made Firefox 4 not seem much faster than version 3.6 for page rendering. Once I updated my video drivers the performance improvements where definitely noticeable.
With my video driver updated I gave Firefox 4 a good workout on dozens of web sites I frequent and I’m very impressed with the rendering and JavaScript speed improvement over version 3.6. However as I played around with new features I hit a major crash problem that nearly made me roll back to 3.6. After logging out of Google Reader, 
and when opening some other pages, the loading icon showed that the page would never finish loading. When I closed the loading tab and then went to close Firefox the confirm close dialog said that Firefox thought I had more tabs open than where visible.
A few times Firefox even got stuck so badly I had to terminate it from task manager. Searching the support site and checking everywhere with Google I couldn’t find anyone else having this problem.
Following normal troubleshooting procedures I first disabled all my extensions and plug-ins to see if any of them was causing the problem, nope the problem persisted. I began to suspect there was something defective in the profile I was using. I’d been keeping this profile through all the upgrades since 2006 so it seemed quite possible it was corrupt in some obscure way. So I fired up my newest and cleanest profile, success, it did not have the problem. With my suspicion confirmed, I backed up my old profile, deleted it in profile manager then made a new profile with the old name. I repeated this for all my older profiles and they all now work flawlessly with Firefox 4. So my second tip is, if you have any problems after upgrading an ancient profile, nuke it and start fresh.
My next tip only applies if you are running Windows XP, turn on the new Firefox Button (it’s on by default in Vista/Win7). It replaces the standard menu freeing up a lot of screen space, see this article for details. Finally you may be wondering what happened to the Status bar, it’s now called the Add-on bar and Firefox 4 is really designed to run with it turned off freeing up more screen real estate.
The old status bar function of showing the target of links is now provided by a small pop-up at the bottom of the screen, very nice. Most plug-ins that put there control icons in the status bar can now have their icons in the navigation toolbar, so most people no longer need to have the add-on bar visible. There are a couple of exceptions, the Xmarks and Zotero plug-ins that for now need to use the add-on bar. To eliminate all the blank space in the add-on bar I’m using the Barlesque plug-in, it shrinks the bar to the minimum size needed and allows you to quickly hide the rest when you want to free up even more screen space.
Now that I’ve used Firefox 4 for a few days I’m comfortable saying this is the best Firefox ever! I recommend everyone upgrade today.
Paul Hutchinson's Blog 