This is a bonus to the LegacyFox blog post—read that one first.
If for some reason (like wanting to use your distribution's prepackaged Firefox) this won't cut it for you, there is a more elaborate and less reliable way. This method also worked with regular (non-ESR) versions of Firefox up to v64, but using such an outdated browser is not recommended. In June 2019, two high-risk security vulnerabilities have been disclosed, so yeah.
First off: This is a stupid idea. Mozilla prevents you from doing this for good reasons. But here we go:
Let's start off by setting some values in
about:config. There is a file that we can use to automatically set them for us. It is located in Firefox's profile directory, which is usually in
~/.mozilla/firefox/, followed by a random string of characters. If you've got more than one, have a look at
profiles.ini; one should have
Then we need to patch
You'll have to run this script after every Firefox update, preferrably automatically. Make sure, the path in
FIREFOXDIR is correct for your distribution. A big shout-out to Akkana Peck, on whose code this is based on.
This will let your old add-ons run again, but chances are, they rely on some of Gecko's APIs, which may have been removed and some changes to them might be required. Mozilla won't sign legacy extensions for you any more, so you'll need to persuade Firefox somehow. So we need to disable Add-on signing. This comes courtesy of a user on "Hacker" "News". I'm too lazy to incorporate this into the above script (patches welcome!), and I'm not sure if you'll need all those scripts to do it, but it works for me™. Depending on your distro, the paths may vary slightly.
I hate footnotes. But I won't bore/distract/confuse you with my (attempt at) snarky humour, unless you really want me to. Press your user agent's hotkey for
window.history.back() to get back to where you were coming from.
.jar. But then, Firefox and Thunderbird got corrupted from time to time. The cause: windows won't back up
.jars, deleting half the browser/MUA after a system restore operation. Renaming this file to
.jais just such a stupid, but completely legitimate way of working around the operating system. Sounds implausible? Read for yourself!