BTW I understand this, but still thing that ignoring/removing transparency (which is the other major part of this issue) is a bug/flaw. In this case it broke part of the seekbars functionality. (Invisible buffer, and no hover feedback)
Since this addon does not allow to change transparency it should leave transparency as it was and only change the color.
I also had stuff appearing before that shouldn't be there. Now I guess it's because of this problem.
For example this on mozillazine after setting bitmaps to "recalculate all":
