Why I'm (mostly) on Firefox 3.5 even today...
First of all...to those who are crashing out of Fx 4...you have a bad extension, corrupted profile or faulty operating system/underlying computer hardware. You can downvote the comment, but I'll guarantee to you that one of those things is the problem and not the browser itself. And no, I'm not a Firefox "fanboi" or any sort of apologist. I have no affilation whatsoever with the Mozilla organization.
The good news is that Firefox 4 does in fact run on Windows 2000, and it runs quite well. So if you "have" to upgrade a computer running an older (Windows family, at least) operating system, you can, as long as that computer has enough RAM and processor capability to run the newer browser. A reasonably fast Pentium 3 (866MHz+) will still pull it off.
Otherwise...I'm less than totally impressed with Fx4. Call me cranky, say I need to take my pills...but I want my status bar back. (Yes, I've got a status bar extension installed, but something so close to the core of the browser's UI should be a selectable option.) I'm not thrilled about the relocation of the stop/load button to the address bar and don't care for the fact that the Add Ons manager is no longer in a small window of its own. In fact, the only thing I've really liked so far about Fx4 is the ability to highlight a URL in text and have contextually appropriate options (open in new window/tab) appear when I right click said text.
Beyond that...I'm probably going to stay on Firefox 3.6 (or 3.5) with the majority of my systems. (And for those who are wondering why I might be sticking with Fx3.5, I'll tell you. There was this really, really useful little "Image Properties" window available in the contextual menu when right-clicking any image on a web page. It bought the farm in Fx3.6, only to be replaced by a window that seemingly has to process every image on a page before it'll do anything.)
Yes, I could probably figure out how to develop an extension that restores these capabilities.
@ZimboKraut -- it's been a while since I've had anything to do with the inner workings of a Firefox extension, so this is worth what you paid to hear it. (Hopefully not much.) Furthermore, it may well send your browser down in flames, throw all your furniture in the trash, tell the world your darkest secrets, insult your mother or worse--so consider yourself warned. If you look at the package that makes up your VMware extension, there is a declaration within one of the files making up the extension that tells Firefox which versions may safely use it. You may try bumping this value out to something that is greater than whatever it is currently set to. I can't recall exactly where this lives, but some quick Giggling ought to turn it up.
I'll get my coat.