You do not say how you installed Firefox. If you just downloaded and untarred it–I do it that way–you may need to add a firefox.desktop file to /usr/share/applications. The contents of that file is enumerated here. (You will only need to do this once.)
I also had a problem on Gnome when I received my Librem a few years ago. I hope this is fixed by now, but I figured out that there was no gnome-applications.menu in /etc/xdg/menus or in ~/.config/menus. I did a forced reinstall to get that generated:
sudo apt install gnome-menus --reinstall
Good luck!