You can download from here. https://www.mozilla.org/en-US/firefox/all/#product-desktop-esr

Download, unzip, and run firefox-bin from the folder. Running more than 1 browser on multiple devices allows me to have my data in more browsers as a back-up in case something goes boom.

Note that you need to launch the firefox script to ensure proper running environment (this script calls firefox-bin).

Also depending on where you put the Firefox folder (if you use tarball version), make sure the user has read/write permissions for the folder so as to get Firefox updates. If you are the only user then a folder in Home is a popular way.