I use my web browser a lot. And I mean, a lot. (It's not that I necessarily want to, but the browser seems to have killed off a lot of proper applications, and the world is a poorer place for that.)
I've actually switched to Firefox 4. (OK, on Solaris you don't actually have all that much choice.) But normally it takes me very much longer to update to a newly released version of Firefox. Why the move this time? Primarily performance - I've found that some sites (and I suspect Twitter here) can make the whole browser feel sluggish. Certainly since I switched to version 4 the annoying lags I used to have are gone.
There are a few changes in Firefox 4 that really can't be described as anything but negative. That's my opinion, of course, but I'm a heavy user and it has really irked me that for the last couple of decades we've matched advances in computing with compensating steps backward.
The first thing that didn't actually bother me initially but soon got spectacularly irritating was the new "Tabs on Top" feature. (Ahem, misfeature.) What this really means is that the tabs for a page are separated from the page they apply to by a couple of tool bars (in my case, the Bookmarks toolbar and the Navigation toolbar). This makes the user suffer thrice: the interface appears to put the toolbars into the tabs, causing confusion; separation on screen causes a mental disconnect between the page and its tab; and you have to move the mouse further to get to the tabs, making them harder to use. Fortunately you can turn this off pretty easily by right-clicking (on the home button for example) and unchecking the option.
Far worse (because it can't just be turned off) is the switch to tab feature. So you want to open a site, you start typing its address and it appears in the dropdown list. Only if you've already got it open, you don't get to open it, you get to switch to tab - so the browser just goes to the already existing tab. Now, listen people: if I wanted to go to the tab I had already had open, guess what? I would have clicked on the tab! The fact that I'm entering it again means that I absolutely want a new copy. This behaviour is especially annoying when the tab is in another window on another virtual desktop, because it then brings that up (moving the Firefox window to a different virtual desktop in the process). Fortunately there's an add-on to disable this particular misfeature.
Browsing in tabs was a spectacularly useful advance, adding extra scalability to the browsing experience. Firefox 4 attempts to make them less usable; fortunately I've been able to sidestep that (for now).