Unify the `start-$browser-browser` and the `$browser` scripts

Currently, we ship two scripts: one is start-tor-browser/start-mullvad-browser, the other one is firefox/mullvadbrowser. The reason seems to be related to the updater (passing the directory with the libstdc++6 we ship to LD_LIBRARY_PATH when needed).

However, some users might be launching firefox instead of start-tor-browser (or even worse, the actual binary - firefox.real!). This is a risk, because they're missing home isolation and especially the fontconfig settings.

Could we do something to unify these scripts instead?

The first course of action would be to test Tor/Mullvad Browser (and the updater) in an old system, to trigger the need to use our libstdc++.