If you don’t know about smart keywords, basically you can set phrases as shortcuts for specific bookmarks. I want a folder with all of my keywords synced, and that folder only. On each profile or account I have have whatever other folders and the placement of the folder with the keywords doesn’t matter as long as they have the same name.
Does the normal firefox sync not work?
You can use local syncing for your os. Or create symlinks for the folder, but I don’t know of the risks.