>chrome/webkit and Firefox duopoly

Blink (Chrome) is not WebKit. If anything, the duopoly is Blink and WebKit at places 1 and 2 respectively.

Firefox is at around 3% market share. There’s no “-poly” to Gecko at all.

Blink is derived from WebKit, so is in the same family like the other Blink/WebKit derived browsers. Fireox/Gecko is a different browser implementation.