Firefox rarely crashes on me, but, it does have quite the appetite for memory (though, to be fair, I rarely notice or care).
I'm running a fairly fresh instance of FF 3.6.6 on Win7, only one other tab open, and it's averaging about 145MB (it keeps fluctuating from 143 to 148, but spends most of its time at 144MB).
What I find amusing is that I'm also running Ubuntu 10.4 in a VirtualBox VM, running a couple terminals, hi-res wallpaper, compiz enabled with 3d cube rotation, and Chrome with the same two tabs open, and the entire VM w/the OS + Chrome is only consuming 114MB.
I'd bet VirtualBox is using using some more memory that isn't showing up in task manager, such as drivers and the like, but I still find it amusing.