[Feature] Adding instructions to use the AUR package with Flatpak

[iro-miya]

Is your feature request related to a problem? Please describe.

I figured out how to use the AUR package when Firefox is installed with Flatpak.

If I could, I would do a pull request to add the instructions to INSTALL.md, but that file seems to only be part of the AUR repository, so instead I'm opening an issue so you can add it.

Describe the solution you'd like

Make the following changes to INSTALL.md:

1. "Open directory" doesn't work on Flatpak. To install to Flatpak, user should manually cd to ~/.var/app/org.mozilla.firefox/.mozilla/firefox and find the profile directory.

2. At some point, they need to run this command

sudo flatpak override org.mozilla.firefox --filesystem=host-os:ro

(replacing org.mozilla.firefox with whatever version of Firefox they use)

This is needed to expose /usr to Firefox so that it can get to the theme in /usr/lib/firefox-gnome-theme. This also exposes other system folders, but there isn't any other way to do it in Flatpak as far as I can tell. Folders are exposed as read-only to minimise permissions.

3. Finally the system folders will be mounted in /run/host, so when installing on Flatpak, step 5 needs to be this:

ln -s /run/host/usr/lib/firefox-gnome-theme firefox-gnome-theme

Otherwise, the installation procedure is the same.

I just did that and it totally works!

Describe alternatives you've considered
Maybe you could also make it so that INSTALL.md is copied from this repository so that changes can be made on GitHub in the future.

Additional context

[rafaelmardojai]

CC @FunctionalHacker