How to stop Firefox update notifications?

I had the same question!

I assume that you want to completely disable the updates, otherwise you won't opt the option to disable automatically install updates option from the settings, and won't get the notification at all!

Well, actually if you truly want to disable autoupdate, you can add a policy.

To add a policy, follow the steps:

1. Go to the firefox installation directory (for any supported OS).
2. Create a directory called distribution.
3. Change directory into distribution.
4. Create a file called policies.json. And paste this code:

{
    "policies": {
        "DisableAppUpdate": true
    }
}

Save the file, and quit the editor.

5. Restart Firefox if it's already running.
6. To confirm, you can go to the URL about:policies and check if there's an entry like this:

Policy Name     Policy Value
DisableAppUpdate    true

After you are done, from the Menu => Help => About Nightly you get to see this:

When you want to update, you may set the boolean value to false.

Reading More about Policies

For documentation, you can read the options here in about:policies#documentation. You can learn more about how to add policies here.

Hope this helps!

From experimenting with Firefox config, just as a possible addition or alternative when solutions that have previously been suggested here are not enough.

After opening the about:config (typing about:config into the address bar and then pressing Enter), apply either or both of the following config items:

• Search for app.update.silent, then set it to true
• Search for app.update.url, then change it to a fake/non-existent URL

  For example, if the original is https://aus5.mozilla.org/update/6/%PRODUCT%/..., I changed it to xxxhttps://xxxaus5.mozilla.org/update/6/%PRODUCT%/...

This is just extension of previous answer.

Find the user preference file, named as prefs.js, in user specific folder.

• Windows: %AppData%\Mozilla\Firefox\Profiles\xyz.default\prefs.js
• Linux: ~\.mozilla\firefox\xyz.default\prefs.js

Then add these following three lines. These preferences are of the boolean type. If you find any of these lines are already present, replace that with the following:

user_pref("app.update.auto", false);
user_pref("app.update.disable_button.showUpdateHistory", false);
user_pref("app.update.enabled", false);

Also if you want to remove the update URL, add these three lines. These preferences are string type:

user_pref("app.update.url", "");
user_pref("app.update.url.details", "");
user_pref("app.update.url.manual", "");

If these don't work delete/move these three files:

1. update-settings.ini,
2. updater.ini,
3. updater.exe (updater in Linux).

For others, a simpler solution if you are just annoyed by that second type of notification, you can do this in about:config by setting app.update.doorhanger to false.