Yes, it's a bug. Cameron Kaiser refers to it as the stoplights issue. But it's a graphical bug that does not affect operation of the browser so not something he's spent a lot of time with.
Any of my suggestions to deal with it might not be to your taste.
I use Classic Theme Restorer, which is an addon that kills Australis, the GUI style introduced with T4Fx 24.
But beyond that I also use TabMix Plus to put my tabs on the bottom of the screen and I browse in full screen mode. It eliminates the title bar entirely.