Hi Rin, if your device has a "hardware" menu button (usually to the right or left of the central home button), then Firefox will hide the internal button and respond to the device menu button.

If you cannot find a device menu button, do you want to mention what kind of phone it is? Perhaps a volunteer can find a user guide for it.

It doesn't have a hardware menu button. It just has back, home and recent. It's an umi emax mini.

Often the "recent apps" button does double duty as a menu button depending on whether you tap it or long press it (press and hold). There is a report in this support thread that it wasn't working as of November 6th: http://community.umidigi.com/thread-1465-1-1.html

This seems like a catch-22: if Android tells Firefox there is a menu button, but it's not working, it seems you're stuck. Hopefully someone else will have an idea for a workaround.