Comment by IMTDb

At some point; Firefox was marketed as and felt as "the best browser". Now the whole marketing seems to revolve around being the best "ethical/open/private/nice" browser. Real or not; it feels like their number one priority is privacy/ethicality rather than technical excellence.

Being "an ethical alternative to" is appealing to some users; but there is a ceiling to this approach. And there is more and more competition in that space (chromium, brave, or even safari). So the share of people putting "ethicality" as their primary deciding factor is split amongst more option.