@kallekn @osma I haven't checked how Firefish deals with opt-in today - my expectation is that it does what the Vyr/Universeodon search fork, and Eugen's original proposed implementation do, that is federated timeline discoverability would have been enough to enable indexing. Now that Mastodon will be introducing a separate indexable opt-in metadata, I'd expect Firefish and others to switch to that model, too. So, yes, more or less the same.
I don't know if there are any plans for antennae-like search extensions on Mastodon. As you say, if you want them, Firefish exists already.
The global search I'm thinking of is another thing. That's a function of how much the index sees (point 1 above) and how far back in time the data is retained. Search on the scale Twitter used to present is not cheap to build or maintain.