Dev mode is still limited and doesn't grant root access to the device. In dev mode there's still an app certificate, and Apple still signs it, and still prohibits escaping the app sandbox. There are also JIT specific entitlements to allow an app to dynamically load or generate code. Without those, it's unclear how feasible a developer-signed version of Firefox on iOS is.

https://siguza.github.io/psychicpaper/ is a recent writeup that includes more details on (escaping) the limitations.