See this - Remove a device from Sync

Don't use Sync on a computer that you don't own!

Sync isn't temporary access to your data for viewing and for temporary use of you data; your data is synchronized to whatever your device you connect to your Sync account and merges with the existing data. So now even if you disconnect from Sync, your data will remain on that device. And if you would go to delete your data, all the Firefox data on that device would be deleted. So unless you created a new Firefox Profile prior to connecting to your Sync account, where your data wasn't merged with that of your friend. You have a dilemma - remove your data and have your friend's data also be deleted or manually remove your data piece-by-piece.

Note that you should never connect to Sync on a computer that doesn't belong to you.

Sync sends the data from all connected devices to the Sync server and merges this data to all connected devices, so your computer gets the data from your friend's computer and you friend's computer gets your data.