When you control the engine, you can fix problems or at the engine level and not work around them on the front end. When we create new UX, we also create in the engine things like accessibility and l10n support, or new web extension APIs to allow future addons to build on top of our new UX. Example with Split View that shipped yesterday: https://bugzilla.mozilla.org/show_bug.cgi?id=2016749BTW, Zen is a good Firefox fork, so it's based on Gecko ️