User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:7.0) Gecko/20100101 Firefox/7.0 Build ID: 20110908135051 Steps to reproduce: Just open an big image, for big I mean dimentions>2000x3000, file size>6MB (I'm not sure about the exact boundary though), like this one: http://yusa.imouto.org/image/f81ce016143ddf655b7f2113ddbd1e12/test.png Then right click it and check the information. Actual results: You will find that it's not cached, which means you can't save it to your HDD w/o re-download it (it takes time for low speed connection). Expected results: I think firefox should cache files like that in HDD, when the cache dictionary is not very full. Or we should have way to custom this behavior.