Hmm this seems like something you would get if you are using the Firefox for iOS (for iPhone/iPad) version and not the Firefox for Android and Firefox for desktop version. I say this also because you posted this thread with a iPhone.

The mobile version of Firefox for iOS/iPadOS is not using the same Gecko engine as Android Firefox and (desktop) Firefox does as you can thank Apple's restrictions on what browser engine can be used on iOS/iPadOS. https://support.mozilla.org/en-US/kb/add-ons-firefox-ios

Extensions at https://addons.mozilla.org are only available for Android Firefox and desktop Firefox (for Windows, macOS, and Linux).

Firefox for desktop https://addons.mozilla.org/firefox

Firefox for Android https://addons.mozilla.org/android/

Is this about Sync asking to add for devices to the Sync account?

Using Sync is optional. You can close the tab once you have created the Sync account if you do not want or can't add a mobile device. Keep in mind that Sync isn't designed to be used as a backup, but merely is meant to sync data between connected devices. Best is always to backup your personal data locally.