Syncing 2 devices both with existing firefox

Hello, i have run Firefox for years on 2 separate computers each with their own data and now I want to sync them. My problem is I don't understand to the 2 different sets of data if I sync. Do the 2 sets 'blend' somehow? Does one set take primacy over the other (if so, how do I control which one it is?) or do I lose everything from both computers and start again? I've been wanting to sync for 12 months but I am so scared of losing information that I keep avoiding doing anything. Thanks Chris