Blef! Finally found a fix for the awesome Tab Mix Plus Firefox extension and the iFox (and associated) themes! Sheesh. Can't believe I tried to ignore that for so long!!

Follow these steps to fix the problem:

- Open a new tab
- Type about:config
- Create a boolean (Right click >> New >> Boolean)
- Name it: extensions.tabmix.useGreyCloseButton
- Set it as false

And your problem should be fixed.