The Firefox engine legacy goes back to the Maemo times - its not ideal, but what else would you use ? The web engine situation is quite bleak even on desktop Linux distros and its even worse on mobile Linux.
My naive assumption is that WebKit would give a route towards blink and more mainstream compatibility. Gecko was a perfectly defensible choice in the Maemo era, the landscape just moved. That's my point, Jolla sailfish are coupled to old choices and don't have the resource to do much more than keep those maintained.
