It's nice and actually work but it always got disabled everytime I restart the browser.
It's a little pain in the kidney to re-enable it everytime I launch firefox.

With Firefox 42.0, it works. For Firefox 52.8.1 and 62.0, it does not. If images are blocked, pages seems to load forever. If you open the Windows Task Manager, Memory Usage goes up and goes back down again, and continue (sometimes continues to grow forever). As soon as you select Display Images, the page will be loaded with images displayed.

It sucks! Only set permissions.default.image (about:config) to 2 blocks images in a Reader View (F9).