SeaMonkey? Or is there practically no connection left with Mozilla? (I still use Composer for maintaining some basic stuff and use it on my netbook where it ironically is faster than Firefox).
Just one example why Seamonkey is important (for me at least).
So it actually does exactly what you want?!
I can assure you that they do not, because Firefox requires heavy tweaking to run comfortably in 1 gigabyte of RAM.
Seamonkey is real nice though.