Yellow "minimize" in Firefox disappears

I don't know why, but the minimize (orange/yellow) button in Firefox keeps disappearing. It will only come back when I restart Firefox. It seems to be related to me running VLC, although I can't be quite sure. Sometimes it just seems to to disappear for no reason (by disappear, I mean it turns gray and I can't push it).

I have a new Macbook, am running 10.5.6 for OSX, and Firefox 3.0.5.

Any ideas? This isn't the end of the world, but it is REALLY annoying.