On Ubuntu 18.04 and earlier, Firefox is compiled with the GTK Global Menu patch.
On newer Ubuntu releases, Firefox isn’t compiled with the patch. You need to hit the Alt key on your keyboard to show the application menu.
This script will download the latest version of Firefox from Ubuntu 18.04’s repositories, install it and hold its version to prevent it from being updated to a version from a newer Ubuntu release.
You need to have Bash,
To install the first three, you can use
$ sudo apt install bash curl wget
You can check out the script on GitHub.