What I’m looking for is a simple FLOSS add-on for Firefox through which the address bar recognizes Fediverse-style account handles as such, i.e. it infers the URL that corresponds to that profile and opens it. (Or alternatively: It shows that handle on one specified instance, e.g. always from your Mastodon home instance.)

Inferring the URL is of course not entirely trivial, as it depends on what software runs on the instance; but perhaps someone has already put together a solution that works reasonably well… or you would like to make one yourself. Edit for clarification: Even coverage of only some ActivityPub implementations, such as recent versions of Mastodon and Lemmy, would be a great start if support for others can be added later on.

For people with experience developing Firefox add-ons, it would be great if you share insights about what functionalities one should use to execute the redirect.