Not sure why this isn't a built-in feature of every browser.

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!