No idea about most Linux users, but here's what little we know for sure:

Arch pkgstats (opt-in): ~64% FF, ~41% Chromium, ~17% Chrome

https://pkgstats.archlinux.de/fun/Browsers/current

Debian popcon (opt-in): 2.2% Firefox, ~10.3% Chromium

https://qa.debian.org/popcon.php?package=firefox

https://qa.debian.org/popcon.php?package=chromium

Flathub installs: 10kk Firefox, 10kk Chrome, 1.8kk Chromium

https://flathub.org/en/apps/org.mozilla.firefox

https://flathub.org/en/apps/com.google.Chrome

https://flathub.org/en/apps/org.chromium.Chromium

snapcraft statistics isn't public, afaik.