Of course, a halt in FF4 could have been caused by any other piece of software in your system interacting negatively with FF4's memory space. Or it could have been any one of your addons, many of which have a profoundly deleterious effect on the overall stability and speed of Firefox.
I don't have that many addons (only 4, adblock plus, better privacy, down them all & no script). ESET NOD32, my choice of AV protection, is easy on resources.
Ever since upgrading to FF 4.0, my temps has risen, the fan comes on more too (when using FF4). And my CPU usage is higher. I always monitor these things. Too, I keep my HDD defragged with Perfect Disc Pro, a paid program, and other than when running FF 4, there's been no problems. Others has reported issues, too.
Since reverting back to 3.6.16, all is fine.
No offense towards Mozilla, as I know that they strive to produce the best product possible. But with the recent release of IE9, and Chrome, Opera & Safari in the mix of things, they probably felt rushed to release 4. They've fought hard to get where they are, and don't want to lose the ground that they've gained.
I do feel like they will iron things out, as they always do. BTW, even Linux Mint, which in recent months has always updated to the latest version of FF, has still not added 4 to the list yet. It updated to 3.6.16, the same day that the version for Windows was released.
There's too many complaints about FF4 for it to be a coincidence. Some may not have problems, and that's good. When there's less problems reported, I'll reinstall it and give it another rightful shot. It's not that I have anything against FF, for the most part, it's been my default browser since 3.5RC, and I can say that I've been a happy camper.
Cat