Here's my concern with this:

Eventually, all encryption used now will be broken.

This is probably a known, as computer AI/Machine learning will automate finding holes in encryption, and speeds of computers will explode to unheard of levels.

So, someone, probably NSA or equal level, is just holding all the data they can get, for the eventual task of breaking the encryption at a later date, and then a shower of data will become available making even more breaches possible in a shorter period of time. (Humans are nothing if not easily predictable.) They could find similarities in keys, passwords, hardware encryption, PINS, etc. And all your data will already be there for the pilfering.

Now, imagine this on a smaller level. Some P2P server you long since stopped using, still has your encrypted data on it. It, too, is eventually broken. Now your data and footprint are in two places, and maybe that data isn't secure anymore.

Sync, in general, is exposing your data more than not using it would be. I can imagine there's some tap between where Mozilla stores it, and large ISPs, and it's being recorded, encrypted and probably not exposing you, but eventually it will be.

Decentralizing increases your footprint for this type of data capture.