After the initial setup of a sync account it seems to be impossible to change the sync direction. I went to the trouble of cleaning up my bookmarks folder. Took a whole afternoon. After synching it in the hope of spreading the new order to my other devices I found that instead the pre-cleanup chaos had returned to my bookmarks. Well done Mozilla! Of course I have now disabled sync. Maybe I'll have enough patience to have another go at cleaning up the bookmarks folder sometime.