Probably your mobile service provider replaces the original images with ones reduced in size to save bandwidth. It's common practice for mobile providers. So you'd better talk to your provider what can be done. Perhaps they offer alternative APNs.

The images appear low quality because that is what Google sends Firefox. Telling Google that you use Firefox for Android and that their Image search experience is poor on Firefox can be done at the following link

https://productforums.google.com/forum/#!newtopic/websearch