I use multiple devices and it is indeed a pain not to have a server where all the browser metadata is stored.
However, I do not like the idea of leaving all my data in a centralized vault managed by a third-party, even if it is Mozilla.
How about spinning the server part of firefox account management (or part of it) into an independent open-source project, which people could install on their own machines, and provide a setting in firefox to make it point to a custom server instead of the central mozilla server ?