Download latest Firefox tar-ball
First, download a latest Firefox version. The below command will always fetch the latest binaries:

$ wget -O FirefoxSetup.tar.bz2 "https://download.mozilla.org/?product=firefox-latest&os=linux64&lang=en-US"

Extract new Firefox tarball

mkdir /opt/firefox

tar xjf FirefoxSetup.tar.bz2 -C /opt/firefox/

Replace existing binary
Make a backup of the original Firefox binary shipped with Debian and create a new symbolic link pointing to downloaded Firefox executable:

mv /usr/lib/firefox-esr/firefox-esr /usr/lib/firefox-esr/firefox-esr_orig

ln -s /opt/firefox/firefox/firefox /usr/lib/firefox-esr/firefox-esr

Update Firefox
To update your Firefox execute:

$ wget -O FirefoxSetup.tar.bz2 "https://download.mozilla.org/?product=firefox-latest&os=linux64&lang=en-US"

tar xjf FirefoxSetup.tar.bz2 -C /opt/firefox/

Restore Debian's Default
To restore original Firefox execute:

unlink /usr/lib/firefox-esr/firefox-esr

mv /usr/lib/firefox-esr/firefox-esr_orig /usr/lib/firefox-esr/firefox-esr