In the General panel, go to the Firefox Updates section. Click the menu button and select Settings.
We also tried Google Chrome, and that worked fine, too. To prevent Firefox from automatically updating itself after you install an older version, you'll need to change your Firefox update settings: In the Menu bar at the top of the screen, click Firefox and select Preferences. We can report that installing Chromium, then removing the built-in Firefox, installing Firefox-ESR from UbuntuZilla, and then removing Chromium left us with a fully working Firefox 91 with no apparent ill effects. Note, though, that installing actual Google Chrome won't satisfy the requirement. FireFox add-on reviewers have blocked old versions of IDM Integration Module.
You can either let it install Epiphany, then install UbuntuZilla Firefox, then remove Epiphany again, or you can install the chromium package for the FOSS version of Google's browser first. Download Firefox ESR Launch the installer and follow the instructions it. with Apple Silicon and update from an older version to Firefox 84 or higher. This is because other pre-installed programs require a browser to be present. The ESR Version of Firefox wont receive any new features and will continue. Installing Firefox-ESR threw up a different Debian glitch, though: you can't install UbuntuZilla's Firefox-ESR until you remove Debian's version – and if you try to do that, it will automatically try to install the GNOME Epiphany browser. This long-running third-party project packages the latest Mozilla builds of Firefox, Firefox-ESR, Thunderbird, and Seamonkey for Debian-derived distros.
Firefox ESR is intended for groups who deploy and maintain the desktop. One way round this that works on both Debian and Ubuntu is to use UbuntuZilla. Fixed Windows startup crash caused by RoboForm versions older than 7.6.2.
Firefox Extended Support Release (ESR) is an official version of Firefox developed.
You can't install or manage GNOME extensions using either a Snap or Flatpak packaged version of Firefox. This article links to older versions and provides some alternatives. One of these, as the latest release of Ubuntu found out the hard way, is a serious problem if you use the GNOME desktop – which is the default in both distros.