Firefox synchronisation: Can I set direction or reverse changes?

I’ve just set up Firefox synchronisation. Initially on PC with plenty of bookmarks etc. Then on a PC with a new Firefox install and no bookmarks etc. Sadly it has copied the ‘no bookmarks’, set-up from the 2nd PC to the 1st PC.

Is there any way to fix the synchronisation direction? Especially for the initial set-up.

Is there any way to reverse the changes made?