Bring back the option to 'never check for updates'. I don't appreciate the notification to download the latest version popping up and am not interested in updating. I find it irritating.
Open a text-editor, such as Notepad. In there, type:
json { "policies": { "DisableAppUpdate": true } }
Save that file with the name policies.json (make sure it doesn't end-up as .json.txt)
Make sure you have Firefox fully-closed
Go into your Firefox folder (on Windows, usually C:\Program Files\Mozilla Firefox ) Create a folder in there called distribution (unless one already exists; if so, just go into it)
Copy the policies.json file you saved earlier into that folder