Used to run the latest FF on my old ASUS netbook with only 512Mb memory, it was a memory hog easily chewing 200Mb on return back to a blank page and not releasing that memory. Besides startup time, it wasn't all that bad, maybe because the netbook used SSD.
I researched the FF problem once and if I remember right the issue was to do with the flavour of some embedded technology (Javascript interpreter I think) that was architected to run fast but with the penalty of being a memory hog.
Like everything software wise FF has become bloatware - the curse of browser technology is that they have be able to do the latest and greatest while maintaining some backwards compatability.