New #Firefox - How to #bypass the #automatic #updates
1. Install and use instead Firefox ESR.

On ubuntu 20.04 you must add MozillaTeam ppa to your system. On the terminal write:

sudo add-apt-repository ppa:mozillateam/ppa

sudo apt update

Now you have Firefox in you repository and you can install it using Synaptic or from the terminal:

sudo apt install firefox-esr

2. (Radical, I've not tested it, should stop all updates until reversed...)

Write 'about:config' on your Firefof's URL bar (without the quotes)

Write 'update' on the top search bar

Toggle the first line, 'app.update.auto', from true to false (click twice on the arrows at the end of the line).