In its early days, Firefox achieved significant marketshare because it was better and offered useful features that the incumbent browsers didn't.

Nowadays Firefox is just a poor Chrome knockoff with no distinguishing features. As a casual user who switches but is unaware of add-ons/etc, Firefox gives you nothing, so why would you switch?

Firefox can reinvent itself and regain marketshare by shipping actually useful features like built-in ad & distraction blocking, but chooses not to.