I don't give a flying fuck about whether I'm using Gecko or WebKit. Both are open-source.
As for why I'm using Firefox over Safari, it gives me better tracking protection than vanilla Safari, it allows me to use SearXNG, and it means I can set a custom homepage. Also, for some reason, Safari has no private mode on my phone.
Not to mention that Mozilla is working on a Gecko-based version of the browser, as Apple is being forced by the EU to allow sideloading, third-party app stores, and third-party browser engines.