Name: Mozilla Firefox add-ons a.k.a. "AMO"
Homepage: https://addons.mozilla.org
Short description: Accessible with a Mozilla account, AMO allows rating and publishing Firefox add-ons and themes.
Optional API documentation URL: AFAIK there is no public API.

For someone having an account, a public profile is available at https://addons.mozilla.org/fr/firefox/user/<user_id>/. See mine for example : https://addons.mozilla.org/fr/firefox/user/15907231/.
I imagine two ways of adding a proof to an AMO account:

For developers only, a "home page" can be given. It is selectable with a JavaScript selector: const homepage = document.querySelector("dd.UserProfile-homepage");
For any account, the bio section supports abbr tags with title attribute. By establishing a title convention, for example "OpenPGP proof", it would be easy to retrieve the proof: const proof = document.querySelector("abbr[title='OpenPGP proof']");. Please note that this second method would also work for https://addons.thunderbird.net.
The claim could look like proof@ariadne.id=https://addons.mozilla.org/fr/firefox/user/<user_id>/