This looks like #2851 on the COSMIC issue tracker, affecting Firefox. They’ve also reported it upstream.

The COSMIC issue has been closed as having been fixed by 150.0, but there’s at least one dissenter.

Anyway, I’m using a regular Gnome desktop on Ubuntu 24.04, with Wayland, Librewolf 150.0, and funnily enough, for me this only appeared with 150.0 and not before. So now I am affected.

The workaround of toggling widget.wayland.fractional-scale.enabled mentioned in the COSMIC issue seems to work (i.e. after the menus have died, open about:config, toggle the setting from default trueto false, then immediately back to true).