It's called Firefox Sync, for synchronizing Firefox user data across multiple devices. It 's not a "cloud" per se; it isn't intended to serve as backup media as with other "cloud services". The main difference is there is a very limited user interface and basically the only user control is to be able to turn it off (Disconnect) and re enable it. The "connection" is thru Firefox only and there's no way to access your data other than thru the 'Sync' UI inside Firefox.

IMO, the documentation is severely lacking, but here's what we have in separate KB articles.

https://support.mozilla.org/en-US/kb/access-mozilla-services-firefox-accounts

https://support.mozilla.org/en-US/kb/how-do-i-choose-what-types-information-sync-firefox

https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer

If one disconnects, then is all info gone until a new Connect? That would remove sync from the idea of a Cloud-- more like a file transfer.

I (we) have become paranoid regarding (nice, good looking) companies actually giving away a LOT of user data, without telling users.

So, it would be re-assuring if Mozilla had a page on what is saved or not saved, why, and how. Etc.

Thanks for wonderful products.

No, the data which is on the device remains on the device.

Sync - Firefox Cloud Services TOS. https://www.mozilla.org/en-US/about/legal/terms/services/

Data is encrypted as part of the upload to Sync and remains encrypted that data is on the Sync server and is decrypted when it arrives back on a device that is connected to Sync. Dual kA and kB encryption keys are used and no one except for the user ever has access to those keys. IOW, Mozilla can't read your data or used it because Mozilla doesn't have access to the encryption keys.