User Agent Mozilla/5.0 (X11; Linux x86_64; rv:47.0) Gecko/20100101 Firefox/47.0 Build ID 20160304030206 Steps to reproduce: 1. On a device open Nightly 47 and sign in with a Firefox profile. 2. Open some tabs and perform a Sync. 3. On a second device repeat steps 1,2 (use the same Firefox account). 4. On a third device sign in with the same account then open the Sync Tabs sidebar. -> You can see the first and second device name and synced tabs. (OK) 5. From first device go to Menu and click on your account name. 6. On Sync preferences page (about:preferences?entrypoint=menupanel#sync) uncheck the "Tabs", then perform a Sync. 7. On the third device right click on the sync tabs sidebar then press "Sync Now" -> "Turn on tab syncing to view a list of tabs from your other devices" message appears + "Open Sync Preferences" link (OK) 8. From your first device check the "Tabs" from sync preferences page then perform a sync. 9. On third device right click on the sync tabs sidebar then press "Sync Now". Expected Result: First and second device with all their previously synced tabs are synced. Actual Result: Only the tabs from the first device (the one from which the "Tabs" option was checked) are synced. All the previous synced devices are deleted. Note: Not sure if this is the expected behavior or not, but someone can easily lose the synced devices or tabs, if he uncheckes the "tabs" and perform a sync.

This behaviour is expected - the other devices all need to Sync again. I'm CCing Kit as I hope he can help me work on a meta bug that tracks some of these device issues and get some clarity about them.

I'm viewing this bug as a dup of the fact that our data type selections are global and not per device.