1. Type about:config into the Firefox address bar.
2. Search for contentHandlers. You will see a list of values with the following pattern: browser.contentHandlers.types.#.. These should be in sets of threes with the # replaced by a number and the replaced by title, type, and uri in each set.
3. Add a new set of values with the next highest unused number or modify an existing one. If Google or Feedly is one of the existing options, you can modify one of those. The new values should be as follows:
browser.contentHandlers.types.#.title — Feedly
browser.contentHandlers.types.#.type — application/vnd.mozilla.maybe.feed
browser.contentHandlers.types.#.uri — cloud.feedly.com
4. The changes will take effect the next time you start Firefox.