I was thinking of webkit too, but I'm not aware of any browsers expect Safari and the Linux browsers you mention. Neither seem to be cross platform or practical, "viable" alternatives to Blink domination.
- Maxthon - https://www.maxthon.com/mx5/features/dual-core/
- Midori - https://www.midori-browser.org/
I'd say the scenario is similar to Firefox/Gecko and Chrome/Blink. Sure there are clones that use the Gecko and Blink engines, but none of these really are as popular as their original.
I am waiting for another Opera / Presto browser platform to emerge. Till it did, the browser market dominated by Internet Explorer was literally stagnant. Opera with a Presto engine was a game changer with their light-weight and super fast browser engine with feature sets that all other browsers copied (and still lack even today).
edit: FF cycles smoothly (although with a weird white strobing effect with the main blank content area) if I open a bunch of blank tabs, but not if they're actual web pages.
Now if Apple could just fix their damn flexbox bugs and implement web app features...