Hi I have have heard of this issue before, but from what I have been told, it is not really an issue with Firefox for Android. Facebook uses some slightly creepy methods to find out if it should supply a high quality or low quality image to the browser. Firefox for Android blocks this (because it is creepy and we do not like creepy web things) and as a result, Facebook displays the lower quality version if an image. I guess nobody wants this to happen, but we will always try our best to stop creepy things happening on the web and to stand up for users privacy.