How to stop Firefox update notifications?

Anyhow, Firefox keeps alerting me about updates. I have turned the updates off, in about:preferences but Firefox keeps bugging me with notifications like these:

and this:

I don't want to keep getting bothered by notifications, how do I make them stop?

None of the former about:config Configuration changes work nowadays to control update behaviour

This should completely disable the Updates messages in Firefox as per today:

Method 1 - Registry change (Windows)

Create the following Registry Key: HKEY_LOCAL_MACHINE\Software\Policies\Mozilla\Firefox
Create inside, a 32-Bit DWORD DisableAppUpdate, with value 1.
Restart Firefox.

The following can be saved as a .reg file to apply this change:

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\Software\Policies\Mozilla\Firefox]
"DisableAppUpdate"=dword:00000001

Or execute the following line in an elevated command prompt/PowerShell:

reg ADD HKLM\SOFTWARE\Policies\Mozilla\Firefox /v DisableAppUpdate /t REG_DWORD /d 1 /f

Method 2 - Policies.json (Windows / Linux / Mac / Others)

Create the folder distribution at the same place the Firefox executable is located (i.e. in Windows: C:\Program Files\Mozilla Firefox\distribution\).
[Edit: Under MacOS, create the distribution folder under ${APPS}/Firefox.app/Contents/Resources where ${APPS} is the directory you installed Firefox in. This is usually /Applications, but the Firefox app can be installed in other locations (matter of taste mostly).]
Create inside, a file policies.json with this text:

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

Restart Firefox.

Now under about:preferences under Updates, the following label should appear: Updates disabled by your system administrator. and under about:policies.

Other policies can be modified in this way. Check the GitHub link and the about:policies section.

If you ever, ever want the original behavior, just undo these steps.