Latest version of Firefox is a bit buggy. Not all skins applied with WB config skin correctly. Try excluding Firefox in Windowblinds and see if that corrects it.

Not all the skins, just some.

That's looking like a Caption height issue, from the skin code, the Caption height may not be matching the height of the titlebar graphic and if there the skin is older maybe it doesn't even have the "Frame Top Maximized" image, then the skin will use the regular top image, which could reinforce that problem.

Another thing that skinner should keep in mind is that you should not set Top and Bottom sizing margins for the Caption frame, only Left and Right.

Plus tip: in Windows 7 and 8, skins should have the "Window glass areas" all set, right now all browsers need those to work properly

Here is where to set those:

MCMXC2,

You can't only add the values, you need to set the graphics too, did you try that?