you have option to export in BOTH json & html format, which I do on a regular basis. "Backup" stores it in json; then there's an option to export in html.
If for instance, you want to pick & choose certain folders or bookmarks from a BU & add to current list, you can open an html list in any app capable of reading it (preferably w/ editing ability, like Notepad++). Edit / Save what you want, then import or combine those w/ another list. I'm not sure how good Fx is at importing / combining PARTIAL lists, or exactly how it handles importing an html list into the currently used bookmark set.
If when you select import (from html), it only winds up adding bookmarks from the imported list, that AREN'T already in the in-use list. I'm sure there's a KB on it.
But, as I said, you could compare 2 html lists w/ something like WinMerge & have several options on how to combine them, or just find differences in the files. Showing you line by line, where the differences are.
Here's a KB on importing from numerous sources & in different formats. http://kb.mozillazin...mport_bookmarks
I'd BU current bookmarks, then play w/ importing some - from HTML. My impression is it adds list B to list A. I'm not sure it creates dups.
"Restore" (from json) COMPLETELY replaces the current list w/ the BU json copy about to be restored.
I'd ask for suggestions on Mozillazine Forums, & state exactly what your needs are.