Edit 24. May 2019: The accepted answer doesn't work for version 66.0.5 (64 Bit), at least not for me. (And I assume it doesn't work for any other new version.)
I have Firefox 64.0 (64-bit) running on Windows 7 Home Premium.
I'm totally sick of the update nag screen showing up seemingly once daily or even more often.
Even more I'm sick of the FACT that Firefox connects somewhere to the Mozilla website to check for new versions: The Update nag screen info changed to suggesting to update to 65.0 from suggesting to 64.1 before.
What I have done so far: I have read MANY webpages about this topic, and I have changed nearly all http:// and https:// addresses in about:config
My Question: Is there a way to stop the update nag screen?
(Bonus question: Is there a way by modifying Firefox through the UI, through about:config, with notepad++ or with the help of for example winrar to modify the .jar files from stopping Firefox to connect to Mozilla's update servers to check for the newest version.)
(If NOT, then Bye-bye Firefox.)
And btw, I'm surfing on a small mobile MB-contingent of fast data transfer, so hitting the update button once accidentally could simply either stop me from using the internet probably for up to a month, or COST REAL LIFE MONEY for more MB quota.
09.May.2019 Look out!! I manually downloaded the Firefox 65.0.5 installation file, and then let it run: It didn't carry over the distribution directory and the policies.json file in it!!! What do they think?