How to prevent automatic update to Firefox 57.0 Quantum?

The solution to disable automatic updates without running Firefox is as follows (OS is Windows):

• Edit the file prefs.js, located in %APPDATA%\Mozilla\Firefox\Profiles\xyz.default\ which usually translates to C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles\xyz.default\ (Note: username and xyz will vary)
• Make sure the line user_pref("app.update.enabled", false); is present in the file; if it isn't, add it or modify it. Ignore the warning that this file should not be edited
• Save and close the file
• Start and use Firefox without having to worry about automatic updates
• Optional: Note the button "Check for updates" in the "About Firefox" dialog box. Of course, DO NOT click it until you are prepared and ready to receive the update

An alternative solution to blocking all updates, is to move to a fork of Firefox which will keep the XUL add-on format, now called "Legacy" by Firefox, while still installing security updates.

Two Firefox forks that keep the legacy add-ons are :

• Waterfox (the one recommended by Mozilla)
• Pale Moon

I have tried Pale Moon, and it very nicely imported all my add-ons and profile data. Screen layout was lost and so some small customizations were still required, but in general this is now an exact duplicate of my Firefox version 56, except that it still does security updates.

Better off changing user_pref("app.update.mode", 1) to 2

WHY

app.update.mode uses an integer value (default 1) to decide the "behaviour" of updating... if you modify this value you are not:

1. altering the developers code by injecting your own custom code or
2. stopping upgrades altogether

the following is from https://wiki.mozilla.org/Software_Update:Testing

INTEGER - app.update.mode Default: 1 defines the policy by which background downloads are done, and the amount of user prompting that is required: 0 - download all update types (major/minor) without user intervention, regardless of incompatible extensions installed (they should just be disabled after the update restart) 1 - download all update types (major/minor) only if there are no incompatibilities with enabled extensions, prompt with UI otherwise. 2 - download minor updates only, prompt for major updates, regardless of whether or not all enabled extensions are compatible.

an upgrade to quantum is definitely a major upgrade...

Open your Firefox preferences and use the built-in search bar (present since version 56) to look for "update", then check the option to never update automatically.

That's not recommended though, since you won't get security updates. You can use the latest long-term support release instead: https://www.mozilla.org/firefox/organizations/

In Windows 7 I went to AppData\Local\Mozilla\update. In the update folder, delete whatever is there. Also never check your current version in “Help about Firefox” as this will get you back on the the update treadmill again.