The first line of the title gets interpreted by Lemmy’s server software as the title. This detail is displayed nowhere when posting from other platforms.

That's for people to figure out who decide to tag a Lemmy community. Mastodon users cannot just assume everything using the ActivityPub protocol is Mastodon or works exactly like Mastodon. Mastodon displays this community as "Group", not an individual profile, so it's not like this comes out of the blue.

It's Mozilla's fault that there is currently no dedicated official Firefox profile, only @mozilla@mozilla.social.

By the way, your image isn’t visible outside of Lemmy.

Please report this bug then: https://github.com/LemmyNet/lemmy/issues/