Disable or control upgrading of Firefox

You can disable automatic updates. It seems not to be in the settings, so you need to go to about:config (you write that in the address bar). There you find "app.update.auto", and switch the value to false.

EDIT: Even with the value being false, FF still forces the restart (version 67.0.3).

apt-mark hold firefox

That command will prevent Firefox from updating. (if you installed via apt)