Since Firefox's automatic bookmark backups are named
bookmarks-yyyy-mm-dd.json and its manual backups are, by default, named in the same fashion, I'm not sure what would lead you to try to restore from
bookmarks.html. I sure hope you didn't take from my previous post any notion that I was suggesting such an approach!
I was attempting to direct kevs to a Firefox support document, rather than to suggest taking action on the basis of "a wild guess," but it seemed clear to me from his reply that the problem is that since he "need[s] to keep the current set," as he "change[s] things all the time," he's
not looking for a solution which requires overwriting the existing set of bookmarks with a previous set.
I therefore offered a workaround involving
exporting, not backing up, to an html file—which Firefox names, by default,
bookmarks.html—and
bookmarking, not restoring from, that file.
In any case, were he to attempt to overwrite his current set with one of the automatic
bookmarks-yyyy-mm-dd.json backups—or with any other file, if he selected
Choose File... instead—he'd first be presented with a dialog stating
This will replace all of your current bookmarks with the backup. Are you sure? 
