All software becomes bloated eventually, as developers must find increasingly useless things to add in order to justify new versions of the software.
The computer industry takes for granted that all software must be updated all the time. The notion of a product that works just fine and doesn't have to change never enters anyone's head. And users are hypnotized into believing the same thing. I have to wonder if the industry is ever going to change. I'm glad that most other industries (including aviation) don't work this way.
I installed Firefox 4 on some machines at work to test it. It was not an improvement, but it didn't seem to break anything, either. Eventually I installed it at home. It uses a ton of memory (more than half a gigabyte, although most of it is paged), and starts multiple processes (but so did Firefox 3.x—all those "plugin-container.exe" processes). Other browsers do the same. They are all bloated these days. I didn't seem to lose anything with Firefox 4, but I didn't gain anything either. I see no performance improvement.
The user interface has changed again, which is inevitable when software is written and updated by unpaid teenagers. The changes were unwarranted and further conflicted with Windows UI standards. The hourglass disappeared ages ago and the kiddies refuse to put it back; it's still missing in Firefox 4.
I only use Firefox because the alternative (MSIE) is worse. Other browsers like Opera and Chrome aren't even in the running.
All in all, a non-event. Like all such non-events, it requires more resources to run.