Hi

Firefox Sync is designed to do just that - sync passwords, bookmarks and other data between copies of Firefox.

You may also want to check to see if your passwords and bookmarks are now on your friends device.

You would think there would be at least a prompt to ask me if I want to sync local stored passwords (not linked to any account) that are not even mine right? I did not even click on sync. it's just so weird to me that that can happen. so If you would login to my browser for example you'd have my passwords and I have yours. where is the logic in that?

This might refrain me from using the browser as a whole which would be a shame because I like it allot.

As said it's sync regardless of Firefox computers running the browser. Now you have to change your login and hopefully your friend doesn't get excited to see what's in your closet. Let this be a learning lesson never login in on a computer not yours or has separate password protected accounts.

My apologies, i taught that it synced to the cloud and then back to my devices. This was not explicitly told when I activated it. It only syncs from a device to another without the cloud.

My bad for the misunderstanding.

Did you disconnect Sync on your friends computer as in that case Firefox might have asked whether to remove synced data ?

Yes I did this immediately, my problem is solved now

Note that Firefox doesn't sync directly to other devices, it uploads data from all connected devices to the Sync server and merges that to the other connected devices.