How works Firefox Sync?

I would like to know how Sync works.

Is it only a synchronization tool or is it also a backup tool?

How can I be sure that no data will be overwritten? For example, I have a PC1 on which I have 100 bookmarks. I have a PC2 without any bookmarks that I have just installed and I want to synchronize it with PC1; how can I be sure that I will have the 100 bookmarks on PC2 and not, on the contrary, that during the synchronization I will end up with no bookmarks on PC1?
Basically, is there a synchronization direction, a kind of reference profile to ensure that data is not overwritten?
Thanks for your light! :slightly_smiling_face:

Anyone who knows Sync and could explain how it works? answers at least one of your questions.

I’d also like to better understand this.

If you don’t get an answer here, you might have better luck at

One thing that I noticed (the hard way) is that if you use several devices (or Firefox Profiles) and it has been a while since you used some of them Firefox Sync will purge them.

It would be also great to know what the exact conditions are for Firefox Sync to automatically remove the contents of a device/Profile.

I also opened this thread, which is somewhat different, but may be of interest to you: