No right-click menu opens from Firefox's title bar

I find that only with Firefox, the Pantheon window-manager menu (with Minimize, Maximize etc) doesn't open.

This prevent eg moving the application to different workspaces. Double-clicking the title bar maximizes and unmaximizes, and clicking on the icon on the launcher tray minimizes, so these functions are fine.

I have reproduced this on other systems, though it may only occur on higher-resolution screens (eg 3840x2160).

Thanks!