Artifacts from previous tab sticking as background of UL on some pages.

I am a website frontend developer. I have a UL on a development site that was fine in all browser testing and FF 7.0.0. When my FF updated to 7.0.1 I noticed a strange color behind the UL menu. The color was actually artifacts from the tab that was open before switching to that site's tab. I can recreate this on multiple machines in the office. The area that is being affected has a black image background. The transparent image on the background of the LI is still transparent but is now laying over the artifact area. Here is an example screenshot: http://juggopop.com/wp-content/uploads/2011/10/screenshot-ff-bug.jpg