Sync: What do you do if you change your main device?

Hello I assume that some time in the distant past I logged in from my main device to set up sync and then with my laptop and years later my mobile phone The problem is that I changed my main device a few months ago and although I can log in to Firefox with it, I can't see how to name it as my main device. I did manage to get my folders bookmarks onto my main device but since then I've added and now I need to sync this over to my laptop and mobile But how to I make my new computer my main device? Thanks