To deactivate synced devices:

1. Open the Sync options window.

2. Click on Deactivate This Device. A prompt will appear.

3. Click Reset All Information to confirm. Your device's data will no longer be synced with the server's Sync data and you'll be logged out of your Sync account.

For more Information on Adding, renaming and deactivating your devices please follow the link Managing Sync Devices

Thanks for your answer, however my problem is – maybe it was not clear enough – that I don't have the device already. So is there a way to deactivate a remote device from Sync which is connected to your account? Imagine how important it would be in the case of a lost device as well...

Hello lentemarton,

I hope this one will help you, please read the article carefully it describes how you can deactivate sync from Lost devices.

Disable Firefox Sync on a lost phone or tablet

Thank you! It really works, and it seems the only way, however it is not an elegant solution to say change all of your account passwords to solve that. I think it would be a necessary function to have a list of all your synced devices and to be able to manage them in Firefox. I hope this function will be added soon! I have noticed that not I am the only person who miss this opportunity.  ;)

I had my tablet stolen (before setting up sync) and just deleting the device would have been ever so much more appropriate. When something like that happens you have a whole mess to deal with - messing around with sync just adds to the anxiety!

I found you can see how many (but not "which") devices you're syncing. Type "about:config" in the address bar -> click the "I'll be careful..." button -> type "numClients" in the Search field.

This number comes from the sync server and will show you how many devices are in sync.