Firefox being “heavily advertised (or astroturfed)” on social media isn’t something I personally have ever seen in the last decade. (Unless one includes Mastodon as social media, even though its userbase is nowhere near representative of the general public.) And today a substantial number of internet users are mainly using smartphones, and the default browser on that smartphone, and the very idea that one can use a different browser has faded from the culture compared to the early millennium.