As a developer, you can absolutely grab the code and run the app on your own own devices. However, there are a few manual steps that you need to take before this works.

If you want to build the v1.0 app with Xcode 6, then use these instructions: https://github.com/mozilla/firefox-ios/blob/v1.0/BUILDING.md

If you want to build the (upcoming) v1.1 app with Xcode 7 beta, then use these instructions: https://github.com/mozilla/firefox-ios/blob/master/BUILDING....

Happy to help here or in a new thread if you get stuck in the process. We have tried to make it as simple as possible but due to some code signing complexity it can't be a 'download & run on device' kind of thing unfortunately.