Anglefish has been build specially for Plasma Mobile and use Kirigami (the framework powering most of the PlaMo apps) and QtWebEngine (based on Blink).

Firefox sadly doesn't give us any ways to just use the web engine and do our own UI on top of it. It's a bit sad because Firefox is my preferred web browser on the desktop.

One could guess it's because Plasma Angelfish want to be well integrated into the rest of the KDE ecosystem (works well with Plasma, has KDE wallet integration and all that jazz). I think Angelfish is based on webkit (via Qt) as well, for better or worse.