Firefox for iOS is a browser from Mozilla, for the Apple iPhone, iPad and iPod Touch mobile devices.[5] It is the first Firefox branded browser not to use the Gecko layout engine as is used in Firefox for desktop and mobile. Due to iOS security restrictions chosen by Apple (specifically the inability to set writable pages executable, which is essential for just-in-time compilation), Firefox has to use the built-in iOS WebKit-based rendering framework instead of Gecko.
For Firefox Focus, which is a fairly young (released last year for Android) offshot. The main Firefox for Android has been using its own engine all along.
Firefox for iOS is a browser from Mozilla, for the Apple iPhone, iPad and iPod Touch mobile devices.[5] It is the first Firefox branded browser not to use the Gecko layout engine as is used in Firefox for desktop and mobile. Due to iOS security restrictions chosen by Apple (specifically the inability to set writable pages executable, which is essential for just-in-time compilation), Firefox has to use the built-in iOS WebKit-based rendering framework instead of Gecko.
In short - no Gecko.
[1] https://en.wikipedia.org/wiki/Firefox_for_iOS