Works pretty well, but for some reason the context menu doesn't always offer the rotate image option. In the situations where context menu doesn't offer rotate image, Firefox still knows I am right clicking on an image because it offers open image in new tab in the context menu.
Also, I know developer is minimizing impact on context menu, but I think I would prefer 2 options in the context menu instead of 1, rotate clockwise and rotate anticlockwise would be nice, to prevent having to rotate 3 times sometimes to get the orientation correct.

I just had a look at the source code - it is very easy: simply adds the tag data-rotation to an html-img-tag and increases it every time about 90 degrees. It should be that easy - thanks!
If the image gets crop this depends on other html-Elements containing the image. Simply open the image in a new tab and rotate it there.

Ich habe mir den Quelltext angesehen - es ist sehr einfach gehalten und fügt einfach den Tag data-rotation hinzu, erhöht diesen jeweils um 90° - genau so einfach sollte es sein :-)
Wenn ein Teil des Bildes abgeschnitten wird, liegt das an DIVs oder ähnlichem - Bild einfach in neuem Tab öffnen und dort rotieren

the problem is, it just cuts parts of the image it rotates. If you rotate left a part of the image is just gone. It should be centered and then you can see the whole picture

As a Waterfox Classic user, this does exactly what I needed! Thanks!