firefox esr is packaged in stretch but I like to run the latest release. Thanks to the debian wiki, I can. This is how I do it.
firefox-wx.y.z.tar.bz2 will download to ~/Downloads folder (currently firefox-62.0.2)
navigate to your Downloads folder
unpack the latest firefox into /opt
$sudo tar -xjf firefox*.bz2 -C /opt
this places firefox in /opt/firefox/
create a desktop file
$ sudo nano /usr/share/applications/firefox-stable.desktop
paste the following into the newly created file:
X-GNOME-FullName=Firefox 62.0.2 Web Browser
The newest version of firefox should appear in the internet start folder along with firefox-esr
Delete the downloaded file from you downloads folder
In the future when a new version comes out, download it then
$ cd Downloads
$ sudo tar -xjf firefox*.bz2 -C /opt
You can also edit /usr/share/applications/firefox-stable.desktop to update the version number to whatever you downloaded. The debian package manager will still manage firefox-esr normally and you can always access it in the normal ways.