A blog entry at the Mozilla website describes the roadmap for "upgrading" Firefox users to the new sync version 1.5, still without any functioning support for self-hosters.

In practice, that means that users will be forced to use Mozilla's own Firefox sync infrastructure which includes authentication services as well as the actual sync servers. The current sync version 1.1 allows administrators to self-host the sync servers. Such an option, while allegedly being developed for sync 1.5 also, is not available for that version yet and won't be before the forced migration.

Forcing users to upgrade to a sync system for which not (yet?) any self-hosting option is available anymore is - from a privacy point of view - hard to understand and frankly a questionable decision, given the experiences from the NSA scandal. The Mozilla Foundation is located in the USA, thus by law, all US agencies have access to the stored information and the stored information is governed by the almost nonexistent US privacy regulations.

In summary, using the Firefox sync function in the future has to be considered a severe security and privacy risk and must be strongly advised against. Additionally, privacy-aware users might want to think about generally moving away from Mozilla products as clearly the Mozilla Foundation does not have the users' true best interests at heart anymore.