Prevent Firefox from updating, for real this time

Question

I don't want Firefox to silently download and install updates. Their documentation is useless in this regard, as the process outlined there is impossible due to the lack of the buttons and other UI elements in the Settings menu.

For that matter, I don't want Firefox to ever go and browse the Internet on its own at all. Not for updates, not for reporting anything. Nada. I'm so disappointed this isn't the default behavior...

Anyways. I looked into every possible option related to updating in about:config, removed what I could, set timeouts to the maximum value etc. This had no effect. I found a bunch of answers offering to put this or another configuration file somewhere, none of them had any effect so far.

The only way to get back the version of Firefox I want is to do an equivalent of apt-get install --reinstall firefox.

Unfortunately, I'm locked into using Firefox due to plugins, for navigation that don't work in Google's browser, and don't seem to work in Seamonkey or Iceweasel.

I'm on version 90.0. On Linux.

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Screenshots showing settings and configuration

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Firefox policies installed but useless

Below are the screenshots showing that the policies file was added where it was expected to be, and that it had no effect.

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Goodbye Firefox

So, this is an update to say that Firefox, despite all the efforts above, decided to go to the Internet on its own and essentially, just spy on its user. Sad, but this means uninstall until better days. It's sad to see the browser that was a symbol of resistance to the corporate evil that spread through the Web to become the corporate evil that spreads through the Web.