Tenfourfox Bug?

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.

 

I get that issue exiting out of fullscreen mode on a YouTube video for example. My late 2008 MacBook Air running Snow Leopard did the same on the official Firefox, until it was fixed in one of the more recent versions.

The most simple fix is to enable the title bar.

 

enable the title bar, i.e. let the browser show the name/title of the side visited.
Back then after installing classic theme restorer and still having cut traffic lights now and then, that was my permanent fix, I happened to stumble over and I was satisfied.