How do I sync two bookmark folders together? So that when I paste a bookmark in one folder, the corresponding folder will get a duplicate of that bookmark also?
I have a bookmark folder within a folder which I believe belongs also in another folder. I want to duplicate it, but I dislike the idea of having to manually post the same bookmark in the now seperated twin folders twice from the moment I split them. Is there any way to sync the two folders so that a bookmark that is posted either one or the other is automatically duplicated in to the other folder that folder is synced with?
No, nothing like that is built into Firefox. And using the word "sync" in that manner is confusing; Sync is a totally separate feature that synchronizes Firefox data across multiple devices that are running Firefox. https://www.mozilla.org/en-US/firefox/sync/
Now, as far as what you are asking about where the same bookmark is duplicated in multiple folders - NO - Firefox doesn't have a feature like that for saving new bookmarks. In fact saving the same bookmark multiple times in different folders was "removed" from Firefox all the way back in Firefox 3.0 in June 2008 when the Tags feature was introduced. https://support.mozilla.org/en-US/kb/categorizing-bookmarks-make-them-easy-to-find
It is possible to copy a bookmark into a different folder so it appears to be saved in both folders. And it is possible to save an actual duplicate bookmark into a different (or even the same) folder via a drag'n'drop operation, by dragging the website icon from the Tab or the Location bar into the folder where you want it saved. Works best when dragging into the Bookmarks Sidebar, but dragging the icon to the Bookmarks Menu Bar menu item will open that drop-down menu.
Gets tricky though when trying to open individual folders in the drop-down and gets worse as deeper levels of folders are attempted; too easy to "overshoot" the folder you want it placed in and then have the entire drop-down menu just close. leaving the user to start all over again with saving it. Same for dragging to the paired Star / Clipboard button on the Navigation Toolbar; hard to target the exact folder you want when it is below the main level of folders and made worse with changes made in Firefox 48 where "Recently Bookmarked" area was expanded into the main drop-down listing.