firefox bleeps for a facebook chat msg but it does not show it

hi, every once in a while firefox bleeps as when i receive a chat message in facebook, but the chat does not open and i can't figure out who's been calling in. this does not happen very frequently, and the chat seems to otherwise work fine. i'm running on fedora 14, ff 7.0.1 the problem has been around for at least a month or two, so unfortunately i can't exactly determine if it was generated by upgrades in ff or fb. it was present also with ff 6.0, but as far as i remember not on ff 5.x