Support Firefox in Flatpak (if possible)

[NoTuxNoBux]

In a similar way to #325, it would be nice if this could work with Firefox that is running in a Flatpak, if at all possible.

[FiloSottile]

Sure, we just need to know two things: where the profile folder is, and where the firefox binary is.

[teohhanhui]

where the profile folder is

~/.var/app/org.mozilla.firefox/.mozilla/firefox/

where the firefox binary is

~/.local/share/flatpak/exports/bin/org.mozilla.firefox or /var/lib/flatpak/exports/bin/org.mozilla.firefox

[knilink]

adding ~/.var/app/org.mozilla.firefox/.mozilla/firefox and /var/lib/flatpak/exports/bin/org.mozilla.firefox works for me