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On Ubuntu this is very easy since it comes with a snap of Firefox already.
So for Ubuntu changing to the ESR version is just opening a terminal and running some snap commands.
Open a terminal and list your snaps:
snap list
Now lets get info on firefox:
snap info firefox
Now lets install the ESR channel:
sudo snap refresh firefox --channel=esr/stable
When it completes the ESR version of FF will be installed (check with the help-about section in FF):
It is very easy to use snap as you can see. For Debian we have to do a bit more as snap is not installed by default.
So in Debian lets open a terminal and type: