This is a Firefox extension to hide Elo ratings on Lichess.
Lichess has since added a user preference to hide ratings, making this extension largely obsolete. However, the extension can toggle ratings on/off with a single click in the address bar. The Lichess user preference requires you to navigate to the preferences submenu, which is not what you want during a game. On the other hand, the Lichess preference will (probably) always work, whereas the extension can be broken by (even invisible) changes to the Lichess website.
For now I will keep the extension available, but if (or rather, when) Lichess makes breaking changes I may unpublish it.