It's not working at the moment (3 March 2025, Firefox 135.0.1). The extension's GitHub repository was last updated 7 years ago, on 26 Dec 2018. Hence, it's safe to assume this extension will never work again.

The development of this add-on has stopped, because the creator of this add-on has passed away, as it is stated in the creator's user profile.

For a good replacement, search for the "Format Link" add-on (by Hiroaki Nakamura).