for me it's Tab Groups (actually one of the Extension the author wrote). If that is gone, there is no good reason for Firefox over Safari

Yup, spacebar toggles play/pause :) ... Plus many more key bindings

same here :(

Yeah ... finally a new packaging system. I think it's always good to start a new packaging system as there are only about 5 gazillion ones out there. A least they have enough sources they can look at when they run in the same problems that everybody else has run into before. Writing a new packaging system seems to be the new Hello World.