If you have a recent bookmark.html file, you might have created it manually or maybe you have an extension that created it.
For quite a while now, Firefox has made its bookmark backups in JSON format. While there's no harm in making extra backups, if you only intend to restore them in Firefox, you probably won't need the HTML format. It's less complete than the JSON format (e.g., no tags).
If you want to back up bookmarks off disk, try copying the places.sqlite database file (contains bookmarks and history) or the bookmarkbackups folder (contains dated daily JSON backups).