You will not be able to install Firefox using your system package manager, as it won't be available in your FSDG-respecting repository. To get around this you need to add a repository that includes Firefox, but doesn't interfere with your existing packages.
You probably don't want to add the repository of a whole distribution, because that almost certainly will interfere with your existing packages.
A simple way to avoid this is to use a separate package manager that has access to an independent repository.
Assuming you're running Trisquel, PureOS or another Debian-based distribution you could try these options:
Snap Store (via Snap package manager)
sudo apt install snapd
snap install firefox
snap run firefox
Unofficial Firefox repository (via Flatpak)
sudo apt install flatpak
flatpak install --from https://firefox-flatpak.mojefedora.cz/org.mozilla.FirefoxNightly.flatpakref
flatpak run org.mozilla.FirefoxNightly --no-remote
(Firefox should eventually appear in the Flathub repository, but isn't there yet (1))
For guidance on which package manager and repository is best for you, either ask a concrete question here ("How can I access repository X from distribution Y") or post a more open-ended question on Mozilla Support or your favourite discussion forum.
(Your distro discussion forum will not be suitable place to ask for advice, since its purpose is to dissuade you from installing non-free software in the first place.)
Note that adding external repositories may mean that new software packages show up in your software GUI (e.g. GNOME Software), including other packages that are non-free. This is a side-effect of adding non-compliant repositories to your FSDG distribution. From now on you will need to manually check the license field of an application before installing it, if you want to avoid installing any other proprietary applications.