OPML Support is a Firefox extension that is simple yet priceless because it will allow you to export/import your bookmarks and Livemarks to/from an OPML file. Here is what the author says about the functionality:
When exporting, you can choose to export your livemarks, bookmarks, or both. When importing a file, you can choose to import only the livemarks, only the bookmarks, or all of the links listed in the file. You can also choose whether to import/export links in their folder hierarchy.
This may not be an extension that you need to use everyday but I am sure most people will find it useful every now and then.