Possibly a display artifact... Could you try disabling Firefox's hardware graphics acceleration feature? You usually need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.). orange Firefox button ''or'' classic Tools menu > Options > Advanced On the "General" mini-tab, uncheck the box for "Use hardware acceleration when available" If you restart Firefox, is the issue resolved? The developers have tweaked this feature for better driver support in numerous releases, but it just seems to be trouble-prone.