IMO it is very confusing that the privacy policy about Firefox Sync does not mention the encryption in any way. Currently it says:
If you use Sync, we receive a variety of information to synchronize
your tabs, awesome bar, passwords, bookmarks, browser preferences and
other data across devices.
I mean a less detailed description could not be given of a Sync feature like this one. Obviously you recieve the data - the important thing is how (= in what form) you receive this data.
So just a sentence like “However as all this data is encrypted before it is uploaded to our servers we cannot decrypt and access it.” would be very reassuring.