Mozilla can't provide base images without licensing proprietary software (for the RIL primarily, although there may be others depending on the hardware). Also

Mozilla can't provide base images without licensing proprietary software (for the RIL primarily, although there may be others depending on the hardware). Also the focus for Mozilla right now is on providing new low-end hardware and optimizing for that. It's possible to build and flash onto your own hardware, people on the XDA forums have done it. Testing web apps in the simulator (or just with the responsive design tool) is totally doable right now, but it's hard to get an idea of performance without actually testing on a target device.