I run multiple FF browsers(mostly java apps) for work during the day. No problems here. I run a MS XP*home* with 500megs ram and 2.8 GHz processor(multi threading).
I run WXP Home sp2 at home and it grinds to a halt in about 2 hours, that on a P4 3.2 with a gig of ram. I run WXP Pro sp2 at work and it grinds to a halt in about 30 minutes. P4 2.8 1GB of ram.
It's too annoying to have to kill FF and Thunderbird several times a day.