I use custom skin on a tab. Is the described bug related to RadMenu only. Because if it is not then we might be able to help jwize anyhow.
I have a skin created for a tabstrip (which is working)
I have my own master.master page and added a link to my css files (I have them in ~/css/)
When you use firebug in firefox and check the styles do you see your own styles and are these overruled by SF or dont you see your css styles at all?
It would be good if you could attach a screen shot of the firebug css styles so one can see a bit more what is going on. Maybe even a screencast of your settings as well as debuging in FF. www.jing.com
This will help in helping you.