Use the unofficial Firefox flatpak repository. site 
To run enter:
flatpak run org.mozilla.FirefoxDevEdition
Note: I have created a script, called firefox.sh to run the above.
To update enter:
flatpak update org.mozilla.FirefoxDevEdition
Note: I have created a script, called update-firefox.sh to run the above.
To remove/uninstall enter:
flatpak uninstall org.mozilla.FirefoxDevEdition