The point is that Firefox doesn't store bookmarks as a collection of individual entries—which would make surgically restoring a few folders from a backup easy—but rather as a single database.
You might try
Exporting bookmarks to an HTML file. This gives you a long list of all of your bookmarks, organized by folder, each bearing the name under which it was bookmarked, and each a clickable hyperlink.
That doesn't allow you to move a folder back into your current bookmarks, but you can still easily navigate to each item within a folder. In fact, you could export your current Firefox bookmarks to a file on your Desktop named, say,
October_2_2010.html, then double-click on that file to open it in a Firefox window (assuming Firefox is set as your default browser). Next,
bookmark that page. Now, as your bookmarks change, you'll always be able to access the ones you've got right now, simply by visiting the bookmarked
October_2_2010.html page. (Note that this holds true only as long as that file remains on your Desktop, or wherever you chose to put it when you initially created it using Firefox's E
xport HTML procedure.)
