The other day, I discovered a Firefox nightly using a gig and a half of memory after some light browsing; after some investigation it turned out that had a few hundred meg allocated immediately after startup (having loaded only about:mozilla) and it allocated a few hundred more when I opened and immediately closed the bookmarks and history menus.

Turns out, in Safe Mode it was perfectly well behaved, and after hitting the 'reset all preferences to defaults' button, Firefox not only takes less memory, but is snappier too.