I think it's even slightly different in that Firefox has some dependence on Google (a scary level, actual, if Google ends that deal Mozilla is pretty much fucked) that the fediverse doesn't - the people on the fediverse right now are enough to keep Fedi alive and moving, and I'd find it really, really hard to argue that they aren't there deliberately to avoid being subject to the whims of Meta/Twitter/Reddit, etc. Like, in a lot of ways, it's a sacrifice to be on these services because the bulk of content still exists in the proprietary silos. Because the actual protocols and main developers are also intrinsically motivated by the this separation, it's hard to picture how they could even try to extend/extinguish here.

Like, if Threads fully federates, I'd guess that quite a lot of people block their instance just to keep their hands clean. Those that interact with Threads via Fedi probably fall into the boat that I would. I want some particular content or to follow some people, just not shoveled at me however Meta decides it should be, and not in a way that they can profit from showing me ads. If Meta pulls some bullshit, it's likely the Fedi would more or less just block them entirely then give up and start a Threads account. And I have a hard time seeing a world where they go to Eugen or basically any of the other driving forces in the Fedi and are like "we need you to change Mastodon so we can [do some typical Facebook bullshit" and Eugen are like "yeah cool with me."

I think its more likely that Threads users are eventually going to see fedi users dropping a long comment or some post that is about how it's nice to have a clean ad-free feed and move clients if not over to the fedi in general. It won't be enough to really matter for Meta other than to say "see we don't have a monopoly!" and hey, if the fedi gets a little bigger it's all good for the rest of us.