Firefox backing up.

Firefox backing up.
A bug is bugging me up, but it is unclear to me what is the cause, so no reporting or fixing is yet in sight - whenever I scroll up in Firefox using the mouse scroller there is ~10% chance that the "back" action will be executed. It has happened to me a lot of times now, and I haven't noticed similar symptoms in other programs.
Ubuntu Dapper, FF 1.5, USB mouse, HP nx6110 notebook. Any advise from the lazyweb?