How is this related to Firefox for Android?

It's not. It's Firefox Developer Edition on a MacBook Pro trying to sync witb iOS Firefox on an iPhone.

I'm new here. Obviously, I've negligently posted in the wrong forum. Sorry to have bothered you guys.

You can use Sync. See https://support.mozilla.org/kb/how-do-i-set-sync-my-computer

Thanks, guys, for your interest and attention. Although this question was incorrectly posted in this forum, it's answer may help someone here.

In Firefox Dev. Ed. for macOS X Mojave, I was attempting to sync to Firefox on my iOS iPhone. When I clicked Add Another Device, a dialog popped up asking me to enter a mobile phone number.

I attempted to enter the number in international format, prefaced by my country code, +1. I got an error message -- invalid phone number format -- but no hint as to what a valid format is.

The answer is that:

Firefox does not accept country codes. It only accepts phone numbers from the same country as the ISP is in.

As soon as I entered the number without my country code, it was accepted.

However, that dialog does no actual linking or syncing. All it does is go to the user's appropriate App Store and bring up the proper Firefox app. Once Firefox is installed on the device, one simply logs in and enables sync.