Firefox menu icon is missing on Android - how to access the menu
Some Android devices have a menu button built directly into the device. In these cases, Firefox for Android hides its own menu icon . You can still access the Firefox menu by tapping on the device's menu button:
Where can I find the hardware menu button?
The hardware menu button is typically located at the bottom of the device, next to the home, search and navigation buttons. The button varies in appearance but may look similar to one of the menu buttons below.