I just had a bit of a look and I noticed that the default aero style and the one used in WSB are not the same, there is a missing MUI folder in the the WSB one and it is also a different size 1.15mb for default and 1.11mb for default, I also noticed there was an extra folder in the WSB one called String that seems to contain any modified font properties, so it is clear that the Aero style created using WSB is not the same as the default Aero style and this is why there are some differences as shown by the OP, so this is not a Firefox bug, it is a bug in the Aero style created by WSB.
Another thing I noticed is that when I copied the default Aero theme to the desktop I was able to load it into WSB but it creates an error when trying to test it, I am starting to wonder now if allot of the msstyle errors and strange colour properties being used has more to do with the way WSB is creating the work base for everyone's themes.
Ave made it to where the MUI is integrated directly into the MSStyles itself. And the file size is different because WSB created MSStyles can't share image properties (meaning the property can't use an image already there) so it duplicates them. The String folder is the MUI stuff.
None of these issues logically would have any effect on Firefox.
