Wow such a fast answer! Thanks a lot.
Well meanwhile … I do not know if it was triggered by having time let pass so that something happened. But what I did after submitting my first post I just uploaded those to images (favicon and apple icon) again not using the wizard but the admin settings (branding) and just in that moment I wanted to close the tab just rechecking if login and everything works now correctly … guess what. The firefox mixed content errors where gone!
Wow!
Well @RGJ still thank you very much, I guess this would also fix the rerendering of the optimized images. Do you think it got fired in the meantime because of a time threshold or by uploading the images again using admin panel, not the wizard.
Thanks again, especially @alphanoob1337 for finally getting this to work!
Wow thats pretty inconvenient for a Monday leaving the desk with pleasant happy feeling 