Mozilla have not paid enough attention to the things that make Firefox unique. The add-on API has been neglected and many useful ones no longer work and the authors have no intention of updating them. Why don't Mozilla adopt and fix the popular ones? Why not clean up and improve the APIs to offer a bit of security?
Unique features like tab groups have been abandoned, and I think that particular one is slated to go away at some point. They say no-one uses it but that's because no-one knows it's there. Performance sucks too and it hasn't improved like everything else.
There are no compelling reasons to use Firefox any more, except for a few unusual cases like Tor and obscure add-ons.