I believe the patch on Bug 833511 introduced behavior that is surprising from a UX perspective. I'm running FFx23 on Mac OS X, although I suspect this reproduces on other versions. (1) Adjust your browser window so that it is wider than it is tall. (2) Navigate to a very large, landscape portrait (e.g. http://farm3.staticflickr.com/2833/9407749984_e572f3cbfb_o.jpg). (3) Use the "two-finger twist" gesture to rotate the picture 90 degrees. (4) Note that the image's *height* is now scaled to match the browser's *width*, and bleeds off the top and bottom of the window. A scrollbar appears that allows viewing of the bottom of the image, but not the top.