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

I want to install Ubuntu 20.04 on the old Notebook of my parents because the Support for Windows 7 ended and the machine is to slow for Windows 8 or 10 - and my knowledge ends with Windows 7. I have to keep thinks easy for them, too much updates and missing language support (whats normal with every new Firefox version) is a problem. But i can't find a PPA for Firefox-ESR 91. Is it enough to install the "normal" 91 and disable updates for 92 and newer versions? What Updates are disabled then: only from 91 to 92 or ALL Updates (from 91.0 to 91.1)?

For old notebook you can consider Ubuntu MATE 20.04.4 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 ESR 91 too.

Installing firefox-esr from Mozilla binaries

start the terminal and then

wget https://download-installer.cdn.mozilla.net/pub/firefox/releases/91.1.0esr/linux-x86_64/en-US/firefox-91.1.0esr.tar.bz2
sudo tar -xf firefox-91.1.0esr.tar.bz2 --directory /opt
sudo ln -s /opt/firefox/firefox /usr/bin/firefox-esr

sudo su and create a file firefox-esr.desktop

cat << EOF > /usr/share/applications/firefox-esr.desktop
[Desktop Entry]
Version=1.0
Name=Firefox ESR
Comment=Browse the World Wide Web
GenericName=Web Browser
Keywords=Internet;WWW;Browser;Web;Explorer;
Exec=firefox-esr %u
Terminal=false
X-MultipleArgs=false
Type=Application
Icon=/opt/firefox/browser/chrome/icons/default/default64.png
Categories=GNOME;GTK;Network;WebBrowser;
MimeType=text/html;text/xml;application/xhtml+xml;application/vnd.mozilla.xul+xml;text/mml;x-scheme-handler/http;x-scheme-handler/https;
StartupNotify=true
Actions=new-window;new-private-window;

[Desktop Action new-window]
Name=Open a New Window
Exec=firefox-esr -new-window

[Desktop Action new-private-window]
Name=Open a New Private Window
Exec=firefox-esr -private-window
EOF

If you want to use your manually installed Firefox-esr as the default Alternatives browser (x-www-browser), run

sudo update-alternatives --install /usr/bin/x-www-browser x-www-browser /usr/bin/firefox-esr 200 && sudo update-alternatives --set x-www-browser /usr/bin/firefox-esr

Versions in other languages at the link

https://download-installer.cdn.mozilla.net/pub/firefox/releases/91.1.0esr/linux-x86_64/

Installing snap on Ubuntu

snapd can be installed from the command line(terminal):

1. sudo apt update
2. sudo apt install snapd
3. VERSION firefox 91.2.0esr-1
4. sudo snap install firefox --channel=esr/candidate