It works fine with my Cinnamon window manager. So the bug could be in Firefox, in your WM, in both, or more likely in the integration of two, as a side effect, which why I’ve said “on the edge”. It’s nearly impossible to test your software with every combination of a system. So the solution here to file an issue both for WM and Firefox and hope someone from either communities will solve it. Or just get another browser or WM.