The official browser for SailfishOS is Gecko-based. The SailfishOS browser source code is FOSS, so we can see for ourselves, from https://github.com/sailfishos/

The official browser for SailfishOS is Gecko-based. The SailfishOS browser source code is FOSS, so we can see for ourselves, from https://github.com/sailfishos/sailfish-browser https://github.com/sailfishos/sailfish-browser : > Sailfish Browser uses Sailfish Silica Qt components for the browser chrome and gecko engine with embedlite Qt5 binding.