The name of automatically created JSON bookmarks backups in the bookmarkbackups folder includes a total item count (folders and separators included) and an hash value to prevent saving the same backup more than once. You also see this count in the Library, Restore menu drop-down list.

• bookmarks-YYYY-MM-DD_<item count>_<hash>.jsonlz4.

You can look at this tool to inspect a compressed jsonlz4 bookmarks backup and possibly convert this file to HTML to be able to import/merge bookmarks.

• https://www.jeffersonscher.com/res/bookbackreader.html

I could not get that reader page to load my .json file using either drag-drop or browse. My attempt to copy and paste the missing folder from an older html backup to a newer one just made a bigger mess with lots of duplication. It does say "DO NOT EDIT" in those files, so I shouldn't be surprised.

I was able to restore one of the bookmark autosaves from a few days ago that gave me back that huge subfolder I was missing, so I guess I will just repeat my recent edits on that.

Create a new profile and launch it, restore the bookmarks backup there, then drag and drop the desired folder into your main profile.

Good idea, that would be the simplest way....but

When I tried this I could copy-paste a small number of bookmarks between profiles, but I could not copy-paste or drag-drop anything containing folders. It also seems to be picky about where you can paste to - I could not paste to a folder icon, only to the actual area where a bookmark should appear. I was pasting to the bookmark manager view to keep it from jumping around.

After the edits looked good I deleted my temporary profile and restarted FF. I had a scary moment when it appeared to open a default themed profile instead of my customized main one. Another restart and the theme and tabs of my main profile showed up. Not sure what was happening there.