I think you meant iOS WebKit for rendering rather than Safari. Firefox iOS uses WebKit while it used to use Gecko.
I seem to remember speed being a difference. It’s been awhile. I also switched from gmail to outlook because gmail was using headers and footers for ads which was annoying on a 4” SE.
Thanks, I was wrong about that. On iOS, Firefox switched to webkit for iOS for rendering (which includes JS). For the desktop, Firefox used and uses Gecko; the desktop version of course came first.
Firefox-on-Store uses Webkit but there is an experimental fork using Gecko/Servo in case Apple allows in the future. You can build and developer-sign that fork for your own.
I seem to remember speed being a difference. It’s been awhile. I also switched from gmail to outlook because gmail was using headers and footers for ads which was annoying on a 4” SE.