Seriously? Why would it be the Firefox devs' responsibility to make it work with WindowBlinds? How could I possibly convince them to do anything about it?

Sorry I originally thought you were talking about the black bar which appears on maximized Firefox developer edition windows with a small rounded rectangle for the min, max and close buttons.  This is something only the firefox developers can change.

There does seem to be an issue with some transparent themes which is linked to repainting areas and we have a small tweak for the release version of WB 8.11.

Ah, okay. Glad we're on the same page.

As to hardware/driver differences, there is a tab background image in Pale Moon which displays on two of my rigs and not on the third; interestingly, it does not display on the rig which has the intermittent transparent titlebar on launch.  I prefer the background to not display, so that's good on the one rig, but ugly on the other two.  All three have the same versions of OS & WB.  I mention this again because I believe we should be mindful of machine differences and not expect WB to perfectly account for all of them.  Not saying that's the case for any poster's specific issue.  Just a general comment.