Firefox’s decline was already well in progress before anything changed with extensions. The problem was that Google’s ad profits allowed them to pour money into Chrome without needing the browser to be profitable, and their advertisements of Chrome on the most popular websites were hard to counter. Extensions weren’t moving the needle on that and, as many people involved have explained, the old design was a substantial maintenance cost preventing other important changes to remain competitive with Chrome.

The big question is whether there’s a model for browser development which isn’t subsidized by huge companies which profit elsewhere. Short of government support (antitrust, direct funding, etc.) I’m not sure there is a viable path forward.