Good publicity for decentralization. If they can get it to work, it shows that it’s possible to run a large instance from a non-central host.

Well, sure it's possible. They don't need to prove it.

But I don't think Mozilla has such a large userbase that it would make any difference.

I like the publicity angle but maybe that's for the future. If Mozilla validates the transition and wants to get more involved with Matrix.org then it'd make sense to own and manage their own stuff.

I like that they are being cautious and not jumping with both feet in.

At a technical level, the Mozilla homeserver will be as decentralised as any other server in Matrix: i.e. entirely independent, and federating with the rest of the Matrix as normal.

At a legal level, it's true that the server will be admined by New Vector via Modular.im - but this is basically Mozilla just outsourcing their sysadmin to Modular, while also helping fund Matrix development (given that Modular subscriptions are used to fund development on Synapse and Matrix in general).

That sounds like a benefit for Matrix, but it's not obvious why Mozilla would want to spend time and money on that?

It also keeps their failures untethered to that of a centralized network.

Federation and decentralization are two different properties with different tradeoffs.