Firefox has long supported mailto://firstname.lastname@example.org as a way to handle sending an email to the designated person. To make federation of social media more mainstream, democratized, and user friendly, Firefox should support an activitypub://email@example.com URL protocol.
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`firstname.lastname@example.org` is not an ActivityPub id and has no meaning to ActivityPub. An ActivityPub id is actually just a URI. At-handles are a convenience Mastodon borrowed from Twitter for referencing users and they require a separate protocol, WebFinger, which not all AP implementations will support.