The fact that Mozilla is a US corporation means that it will still have to give the US government the data it asks for, regardless of where the servers are hosted.
A major part of the purpose of persona is to build a system that designs out the capability to engage in surveillance against the users.
They don't necessarily need to move the Mozilla organization, but then they'd need to make Persona an organization in itself and move that organization.
This is why data on Amazon, Google or Microsoft data centers in Europe and elsewhere are still open season for US authorities. The same would apply to Mozilla.