I never understood this. I have come back to Debian after many years (switched to Fedora, Ubuntu, Mint, Kali, KDE and now back to Deb).
These are instructions for installing Firefox directly from Mozilla's website:
- Download Firefox from the Firefox download page to your home directory.
Note: If you want to have a choice of the language and operating system for your Firefox installation, please visit the Systems & Languages download page instead.
Open a Terminal and go to your home directory: cd ~
Extract the contents of the downloaded file: tar xjf firefox-*.tar.bz2
Close Firefox if it's open.
To start Firefox, run the firefox script in the firefox folder: ~/firefox/firefox
If you install a fresh installation of Debian 9, you don't get FF in home Dir., and common sense tells me to upgrade/install where the existing copy is. How I have two versions of Firefox on my station at two different locations.