the addon works fine, but a lot of times, the pictures were not shown, so I had to use private mode anyway.
I have however found a better solution than this addon. Just use temporary containers in firefox. There is an addon that lets you specify which urls will be allways opened in a new container, so that basically means that each time you open medium, it will create a new container with no cache or cookies and then deletes it after you close it. Its the same as private browsing, but you have all tabs in one browser and it remembers history