After further testing I have found that the touch screen jester of scrolling down on a page is being also imputed as a scroll weal jester on the tab bar. This action with tab mix plus causes "Allow scrolling to select tabs" option causes the above behavior. Without this option enabled though instead of switching tabs it will scroll through tab overflow (when there are more tabs then will fit on the screen) like a normal Firefox scroll weal down jester on the tab bar. From this behavior it almost seems that for some reason Firefox 4 is taking flicks to mean both a scroll weal down on the page and on the tab bar at the same time. (this behavior happens in safe-mode as well) Hope this helps debug the problem.