RSS feeds are becoming the easy way for people to keep track of hundreds of Websites in minutes instead of taking the time to visit each site which could take hours. Firefox’s native support is Livemarks which creates a constantly refreshing bookmark to display the news feeds for you. If you start to get a large list of sites and want to move to another feed viewer then it could be difficult. 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.