I can’t speak for Jeroen, but I played around with the upstream builds (both 92 and 91) and my faulty profile a bit and managed to isolate the trigger combination for this: MOZ_ENABLE_WAYLAND=1 and setting layout.css.devPixelsPerPx to my preferred value of 1.2; either resetting layout.css.devPixelsPerPx to its default (-1.0), or running Firefox without MOZ_ENABLE_WAYLAND=1 will make the toolbar items work again. Here’s the graphics portion of my about:support.

So my steps to reproduce this are:

1. start Firefox with MOZ_ENABLE_WAYLAND=1
2. set layout.css.devPixelsPerPx to 1.2
3. click on the hamburger menu button

What I expect to happen:
For the hamburger menu content to show.

What happens instead:
The hamburger menu content flickers or doesn’t show at all.