Comment by rafram

The menus, and what happens when you open/close them, do belong to Firefox. Cocoa gives apps a fair amount of power here. In Firefox's case, the issue is likely because each window specifies its own independent set of menu bar menus - very un-macOS - and it handles that by destroying and recreating the global menu bar every time you switch windows. The stuck menu bug seems to happen when a window is opened or closed, so my guess would be that something about the destroy-and-recreate routine is broken on macOS 27.