The last most security updated version of Firefox on which most old Extensions and Add-ons will work has now been released.
It is
Firefox ESR 52.9
http://ftp.mozilla.org/pub/firefox/releases/52.9.0esr/win32/en-US/
We are in End-Of-Life territory now, in September 2018 there will be a first new security patch which will not be available for versions of Firefox on which old Extensions work.
If you so much as check for updates after September, your Add-ons and Extensions will be permanently gone.
To prevent that from happening, in addition to Backing up your profile of course, here's what you can do:
• You can prevent Firefox ESR 52.9 from being updated, since it is the last Firefox version with June 2018 security updates on which old extensions and Add-ons work. When users click on 'Check for updates' button, they will get a "Firefox is up to date" message even if there are updates available.
Use Notepad to open
C:\Program Files (x86)\Mozilla Firefox\defaults\pref\channel-prefs.js
then simply change line
pref("app.update.channel", "esr");
to:
pref("app.update.channel", "esrDISABLED");
• To remove the 'Check for updates' button altogether from an existing profile, use a text editor like Notepad to create a userChrome.css text file
inside a CHROME folder created where your default profile is. userChrome.css file can be used for customizations. Contents of the file would include:
/* Remove 'Check for updates' button */
@namespace url("
http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
#checkForUpdatesButton { display: none !important; }
