Been using Linux Firefox for over a decade - five years on a toshiba Core 2 Duo laptop with 2 GB ram and a harddisk. Never had any performance complaints even though I sometimes opened >20 tabs.
Could you elaborate on the contents of those tabs? I often have more than 20 tabs open, but even in Chrome this can cause some issues. Nonetheless Firefox seemed worse at it, and perhaps this has to do with the nature of the tabs I have open?
As a university student back in the day I would perpetually have gmail, facebook, university email, youtube open - all js heavy websites. Others tabs were links from HN, wikipedia and/or pdf class notes. Even developed a website for my department at some point, all on firefox.