FireFox does not support beforeinstall prompt. You have to do it manually.
On Android there is a little home icon in the address bar to indicate it can
be added to the homescreen. 

However they keep same code block from chrome to may be support in future check
below:
https://developer.mozilla.org/en-US/docs/Web/Apps/Progressive/Add_to_home_screen#How_do_you_make_an_app_A2HS-ready

https://developer.mozilla.org/en-US/docs/Web/API/BeforeInstallPromptEvent