This add-on (SEVI for short) is designed to improve the experience of viewing stand-alone images (as opposed to images which are embedded in web pages). In particular, the main features that it provides are a choice of background color (optionally with a noise or checkerboard pattern), and allowing for zooming and rotating images by any arbitrary amount. There is also an in-window information display available, in case the title bar is not visible.
When SEVI's zoom mode is enabled, the methods of manipulating the image display include using the keyboard (type h to see a list of all the key bindings), picking commands from the context menu, clicking (to toggle between natural and fit-to-window zoom levels), double-clicking (to remove rotations and reflections), clicking and dragging (to zoom such that the selected area will fill the window), and double-clicking and dragging (to rotate and zoom based on the distance and angle the mouse moves from its starting point and the window center). (Since double-click and drag can be difficult on some touchpads, that last command can also be accessed by dragging during a single-click while holding the control or alt key, or during a triple-click.) If you want to drag (and drop) the image itself, hold shift while clicking and dragging.
Compatibility note: due to the nature of this add-on, it is likely to be incompatible with other add-ons that also alter the display of stand-alone images.
extensions.slightlyenhancedviewimage.frame-script-mode, but that code seemed to cause crashes in some versions of Firefox (so it is disabled by default), and as of 53.0a seems to be completely non-functional. I hope to eventually make a WebExtension version of the add-on, but so far haven't made much progress.