Data Saver option in chrome is a process that routes all data over google's server for "optimization". This, IMHO, introduce a layer of privacy issue that I doubt Mozilla should, or will do. I could be wrong, of course. I understand firefox desktop may have some development features that may block image from being downloaded. If its true, I imagine Mozilla could port that into mobile version, that could save massive data (by not showing images at all). BTW, if data usage is concern, I would recommend using Opera MINI, which pretty much invented data "optimization" years ago for mobile browsers, and has a track record of dealing with privacy issues.