mybirthdaysoon: I had that a few months back. One day it just happened on me, in Firefox.. i had to uninstall FF then install the latest version.. even that didnt always work so I switched to seamonkey.. problem solved then.. Since then I have a completely new system and so far, knock on wood, my FF hasnt had that problem.
So I'm no help.. might get some insite on the Computer board tho as this doesnt pertain to the site structure etc.