twanto, a Mozilla account is not the same thing as a local profile. The local profile's bookmarks etc. are stored on the harddrive.

I suspect that what happened is that you signed into your wife's local profile with your Mozilla account, and your Mozilla account will have no way of knowing that the pre-existing data in the local profile being signed into was not "your" data?

So, to clarify, I did sign into the local profile. But I signed out of her firefox sync account and signed into my own. Why would it transfer over all of her data from the local computer? This seems like a bug, not a feature. It is poor design if that is the intended behavior.

Hi,

Thanks for reaching out. I understand how concerning this is.. syncing should never merge another person’s local Firefox data into your account without explicit consent. What likely happened here is related to Firefox Sync and local profile data behavior:

Local profile vs. Firefox Account

When you signed into your Firefox Account on your wife’s computer, Firefox may have merged the local profile data on that machine (her bookmarks, passwords, etc.) into your account if Sync was enabled and the profile was active.

Firefox assumes that when you sign in on a computer, you want to merge local data unless you choose “Use Firefox account data” instead of “Merge” during the sign-in prompt.

twanto, signing out of a Mozilla account within a local profile should not remove the data stored locally. The bookmarks and passwords etc. will still be there, saved in their respective files on the harddrive.

In a sense you have been lucky in that the pre-existing local data got merged with your sync data.

I have heard reports of people signing into someone else's pre-existing local account and the data in the local account then replaced their sync data. Which in your case would have meant that your own data would be lost, having been replaced in the Sync server with your wife's data.

Because for whatever reason Sync decided that the data in the pre-existing local folder is "newer" than the data in the Sync server, and replaced it.

I think it is best to always have a backup of ones own local profile folder before signing into any new device, and then only sign into a newly created local profile on the device, and not someone else's pre-existing one?