For firefox sync on chrome: It’s completely possible, but the crypto is enough of a pain in the ass to deter most people.

(It’s also going to be very difficult to implement a robust sync system this way, integration with the underlying storage is all but required for that)

In my opinion, crypto is the easy part. I think the only problem I had when I worked on my own Accounts&Sync re-implementation (a crude hack, I admit) was some MAC-related issue with BrowserID, where different (old) Firefox versions had generated different assertions. Don't really remember what it was.

The real problem is data model is significantly different. It's possible, but a lot of boring work, designing the appropriate transformations.

Chrome sync can be also self-hosted, and while I'm not sure how things are today I'd say it used to be significantly easier to self-host.

https://superuser.com/questions/614744/how-to-set-up-a-own-c...