Firefox stores all bookmarks in a database file in the profile folder. To avoid issues such as corruption caused by crashes or accidental modifications or deletions, it is also storing backups of the bookmarks folder in the profile folder.
Interesting in this regard is that it will keep the last 15 backups in that folder, and only remove the oldest one of the 15 when it creates a new.
If you think that 15 is overkill, or maybe not enough for your needs, then you can modify the number easily in the Firefox configuration.
But first of all, let me provide you with information on how to look up the backups.
To do so, load about:support in the browser's address bar and select the show folder button on the page that opens up. This opens the Firefox profile of the current user in the system's file manager.
Locate the bookmarkbackup folder here and open it. Here you find listed all bookmark backups. Depending on how many bookmarks you have saved, they each may have a size of a couple hundred Kilobytes or maybe even Megabytes.
As you can see from the screenshot above, the 15 backups of this particular user have a total size of 2.36 Megabyte. Unless you have a lot of bookmarks, I'm thinking thousands of them, you won't save a lot of disk space reducing the number of backups.
Advanced Bookmark configuration
All of the following settings are handled on the about:config page. To open it, load about:config in Firefox, confirm that you will be save, and filter for the preference names listed below:
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