Or a glitchy plugin; I've noticed Java in particular seems to, on occasion, cause my FF to become unresponsive for a moment.
Or an excessively long RSS feed, such as the one from GiantITP.com (which is over 500 entries long, the owner didn't quite code it so it only returned the most recent 50 or something).
Guess you're getting the point by now, right pmoran? We need more information. Without more, it's all guess work.
So:
Firefox Version?
Plugins installed (ALL of them)?
OS?
System specs (might be low ram for instance)?
An example of a site it frequently occurs on (maybe it's a site related problem)?