The other day I saw a little message on my Firefox main page asking if I wanted to try Firefox 4. I wasn't unhappy with the version I was running, but I said, "What the heck" and allowed it to install. It was pretty much of a non-event. I haven't found anything that would cause me to say it is better, but it isn't worse in any way, either. In the computer realm, I guess we'll call that a win.
One oddity is the BACK button. Prior versions had a little down arrow that showed the progression of websites you followed to get to where you are, and let you jump back in one big leap, not from one site to the next. This feature is now incorporated into the BACK button itself. If you hold the clicker on the BACK button, the progression shows up after a second or so. This is OK by itself, but where before you could go back one step with one click of the BACK button, now it sometimes takes several clicks to convince the thing you really want to go back.
I have not noticed any difference in speed after switching to version 4. On this computer (Windows XP) EVERYTHING is sloooooow. My other computer, with Ubuntu Linux, is blazing fast in comparison. If you want fast computing, and don't rely on any s/w programs that require MS, switch to Linux and don't look back. The difference is like night and day. As an added bonus, you will never see the idiotic "You have unused icons on your desktop" message again!
Steve
Steve