Is your question about the URL for update checks, or the IP address?

For the URL, you just need to allow https://*.mozilla.org/ (I assume you don't need to block pages on any part of mozilla.org).

I don't know whether there is a stable IP address for that service.

You can open the Browser Console to check for update messages. When you open "Help -> About Firefox" then this should trigger an update check that is shown in the Browser Console.

• https://developer.mozilla.org/en-US/Tools/Browser_Console