Of course this is not the experience of the vast majority of Firefox users, so it's pointless to suggest that this means something about Firefox's overall quality. You can easily find people making the same sorts of complaints about Chrome.
Sadly, one of the drawbacks of opensource is that people can take your code and butcher it, and users will blame you.
Some mornings I'm beachballing for my whole first coffee. Eventually I just wind up power cycling the mac if I can't open a shell prompt with which to `pkill -9 plugin-container`....
> pkill -9 plugin-container
Ahhh, ok - that's not FF, that's a crap plugin (likely Flash). It's the browser equivalent of blaming Windows when a crap taiwanese driver gets it stuck. You could try uninstalling all that crap.
Firefox often appears to use large amounts of memory in top / htop, but I believe that's just reported address space allocations. The RES column gives a more accurate depiction of memory usage.