I use FF on OpenBSD as my daily browser. I have at least three different profiles running simultaneously with multiple tabs open in each (though I don't go nuts with tabs like some do -- I never tend to have more than 10 or so open).

It does occasionally seem to get stuck in a state where it just starts chewing CPU cycles and RAM. I've seen this mostly with Google sites like gmail or docs, so I assume it's JS-related. But it's fairly rare, and seems to be improving with each release.