I use a combination of Chrome and Firefox on my computers with a HDD for its boot drive. I do, however, feel confident using Firefox 95% of the time on my one computer with a solid state drive (SSD) for its boot drive.
Maybe, in addition to RAM usage, the read/write speed of the boot drive might be a challenge that Firefox has to overcome as well. I hardly touch Firefox on my netbook with a 5,400 rpm hdd, but Chrome works like a charm.