FF was really crashy/hangy around when Chrome came out—the main appeal of Chrome was that it'd do the same thing but confine the damage to one tab. Also IIRC its dev tools were on par with or better than Firebug (remember that?) early on, and performed way better. FF has gotten a lot better since then so the difference is much less stark. It was worth a little UI weirdness at the time to not have your whole session die when one tab misbehaved.