Okay - I've still been running with Version 90 since this thread - and I disabled automatic updates. However, Firefox still goes out and detects I'm running an "old" version and bugs me with the "New version of Firefox is available - do you want to update?" prompts. Version 92 is the current version BTW.
So I've done some more investigation and you can disable these nuisance prompts by configuring the update policy using a .JSON file.
The steps are here at this link - Option 1 is what I used and appears to stop the nuisance prompts! YMMV.
https://www.journeybytes.com/disable-firefox-quantum-updates/
Option 1: Disable Updates using Enterprise Policy JSON
1. Open a plain text editor like notepad or notepad++ and paste the following code in it:
{
"policies": {
"DisableAppUpdate": true
}
}
2. Save the file as a json file named: policies.json
3. Go to the installation directory of Firefox and create a new folder with the name: distribution. On Windows the directory is:
C:\Program Files\Mozilla Firefox
C:\Program Files\Mozilla Firefox (x86) [if running 32bit Firefox in 64bit Windows]
If you’re using a portable version of Firefox the directory should be:
FirefoxPortable\App\Firefox
FirefoxPortable\App\Firefox64 [if using 64bit Firefox Portable]
4. Now just copy the policies.json file into this folder.