The Android app installer is created by Google and Firefox has no insight to installer errors. That being said the following may help. According to a few sites on the internet a Google Play 919 error is due to some sort of network error. Problem with either your WIFI or data connection. Restarting your phone or trying a different network may resolve the issue. If it does not Google may be able to suggest a solution. https://support.google.com/googleplay/?rd=1#contact=1