So this has been up for a while, but hasn’t gotten much attention. There’s a version-in-progress of Firefox for iOS.
Github repository can be found here.
I couldn’t get the releases to build, but I was able to build and take screenshots of the February 18 pre-release.
I assume they’re using WebKit and not Gecko, as Apple still requires that, last I checked. Either they are, in which case this points to a shift in Mozilla’s policy about using other web engines (they used to be strongly against it), or they decided that it’s fine to write an app and not put it on the App Store, making it just available for Devs and Jailbreakers (which was previously not the practice of mainstream organizations).