Okay after downloading, installing and running on a fresh profile all the Firefox 2.0.0.X builds (2.0.0.0-2.0.0.3), they all behaved the same way. I could have sworn when Fx 2.0.0.0 first came out, all the close tab buttons were red regardless if the tab was active or inactive. The only thing I can conclude is the main theme was updated (yet changes are retroactive) at some point to change this behaviour.