Firefox used to default to black text but due to a bug in firefox itself it ended up not painting at all when not over a tab.
As a fix we override the colour to a mid grey which allows it to usually show, but in your case it is borderline.
There is a css file in the WB install directory (ff.css I think). If you switch to default theme and edit this (you will need admin rights) and then apply your skin you may be able to adjust the colour more to your liking. However lighter works better than darker!
