I wish I could make it but I'm strangling with the userChrome.css.
It's a bit problem to me as with Plasmafox on dark backgrounds (dark theme) I can't see clearly which tab is active/selected.
This because the tab hover/selected color is dark-on-bright but also dark-on-dark.
I can use the "Firefox Color" extension and change the active tab font color but still.
Well not related to the issue the post is about, but you could try to play around with some of the settings under r3dfox configurations. Using different r3dfox color schemes, or disabling r3dfox theme styles or backgrounds for tabs and such. Or force enabling aero fog and stuff.
You could also use the light theme in the browser for the tabbar and co but under general settings have Website appearance set to Dark. So only your tabbar will be light and everything else will be dark.