1. good start, but it misses the most important scaling mode - "no anti-aliasing mode" (many people would like original crispy pixelated images), that can be toggled via CSS code:
html > body > img:only-child {
image-rendering: -moz-crisp-edges;
}
2. it will be perfect if that extension could toggle "no anti-aliasing mode" only when image scaled is bigger than 100% - that way when images zoomed-out (or have resolution less than your display) will be anti-aliased, and zoomed-in images (or those have resolution better than display resolution) will be displayed in original non-smoothed view. If you don't understand that I mean you can check any Faststone viewer software and look for scaling options.

Hi. A good functionality to start with.
However a "Fit image to browser window" option would be nice.
Also for some reason background color doesn't switch (On last Stable and beta FF Windows 10)