You don't !

Firefox sync uses a server but basically all it does is make whatever you choose the same on all devices. Sync is not directional. Sync should add things but not delete them.

• Firefox Sync troubleshooting and tips
• How do I choose what information to sync on Firefox?

Please also take care with bookmarks. If you sync bookmarks it may be good practice to make manual backups. Sync may in some circumstances corrupt or duplicate bookmarks, having a manual backup can make it easier to dig yourself out of the resultant mess.

• Restore bookmarks from backup or move them to another computer

I'm not sure it is actually working that way. I had synced a computer that had my current (at that point) complement of add-ons and settings. Then I worked on my other computer (that was not current). When I went back to computer A, it had "lost" several of the plugins, and most of the settings. It was as if Sync had reverse synced computer A to Computer B's settings rather than from A to B. Hence, it seems that rather than simply duplicating (or adding on) information, sync replaced information.

Hi rowan, Well Sync is not supposed to remove things.

Possibly there is some other explanation. Addons or Plugins just becoming incompatible for instance. If you find you are able to recreate that sort of issue please explain how, as maybe then it would be appropriate to file a bug.

I am not aware whether Sync has any checks to prevent corruption whereby it could overwrite and remove settings. Maybe someone who has a deeper understanding of Sync will chime in. I know it can corrupt bookmarks but that tends to result in additions not deletions. I probably would not be surprised to learn it may also delete or lose data

• See Bug 621584 - [meta] Bookmarks dataloss (duplicated, reordered, moved, lost, attributes changed, etc) caused by Sync
• Please note etiquette

I get the impression Sync is something that is convenient, but not terribly robust, and that it could require major work to improve Sync

Of course if you can reproduce the issue it will be easier to investigate. Maybe set up a couple of new instances of Firefox with their own profiles on one of your computers. Set out with them different and create a new Sync (For obvious reasons do not use the current Sync) just Sync the two new Firefox installs.

(Of course this may even be a known issue there are a lot of Sync related bugs filed a quick search on sync loss lists 4 bugs and two of those are tracking or meta bugs )

I will add an escalate tag to this. Let's see if one of the professional help desk staff is able to add to this. (If they reply it could take three days to a week - please bump the thread next week if no one else chimes in).

P.S. I have also added tags

• syncdata | issues with dataloss
• sync+addons {so addons addon tag added}| Support thread relates to an issue with the addons collections

You maybe interested in glancing at

• Some Sync Updates /forums/contributors/711855

(I am not sure if anyone is tagging threads !?) Modified February 24, 2016 at 3:54:02 PM GMT by John99

2nd edit tagged addon not addons I am trying to clarify Helpdesk's {Rachel) request to tag questions

Thanks John. I appreciate your sharing your knowledge and your suggestions. I will try setting up some different installs and a test account and play around with it to see what happens. May take me a few days though as I am kind of buried with work at the moment.

I am not sure we will really be able to resolve this issue properly. I suspect the most we will be able to do is identify it as being a known bug or add to the existing list of bugs.

On the other hand it is possible you have found some so far unknown recent regression and that will need fixing quickly.