You cant change that Carl, that's default WB behavior, FF uses the same color as windows explorer font.
You may could find a firefox theme with its own buttons however, thats usually how I get around FF issues.
Winestripe real fox 4 and StrataFox are a couple I use, that let the original WB elements be used for things like Tabs and such... not sure if it will help with font colors on buttons tho.
The only way to get around it really I guess, is light buttons in every theme, but thats just not realistic, with some designs.