There is an unofficial Firefox Flatpak repository created by the Fedora/RedHat maintainers for testing purposes: https://firefox-flatpak.mojefedora.cz/
With this FlatPak repository, your profile is stored in <code>$HOME/.var/app/org.mozilla.FirefoxNightlyWayland/.mozilla</code>
If you are on Ubuntu, here is a detailed article on how to install it here: [http://www.omgubuntu.co.uk/2017/02/install-firefox-nightly-flatpak-ubuntu How to Install Firefox Nightly as a Flatpak App on Ubuntu]