Can't find a PPA to install Firefox-ESR 91 on Ubuntu 20.04

Solution 1:

For old notebook you can consider Ubuntu MATE 20.04.3 LTS from https://ubuntu-mate.org/ . Having 4 Gb or greater RAM is recommended for comfortable work.

Modern Firefox versions are fast, but you can install ESR if you like it.

Currently Firefox ESR 91 is in “Mozilla Team” team's Firefox ESR and Thunderbird stable builds PPA, then install using commands below:

sudo add-apt-repository ppa:mozillateam/ppa
sudo apt-get update
sudo apt-get install firefox-esr

with needed locales like sudo apt-get install firefox-esr-locale-it for Italian (see full list in package details.)

Depending on your needs you can have both firefox and firefox-esr installed, or remove firefox package to use firefox-esr instead.

Note: alternative third-party repository is UbuntuZilla, it provides 91.3 too.