Thank you very much for your input. I found the problem today and it was the Firefox version. Checking the errors hinted that html5 and css3 were not supported. Since that seemed really strange I checked the version and found that it was 22.0. I looked that up in the version history and it is from 2013, before html5 was supported by most browsers. At the end of last term Firefox was more or less up to date. In the summer vacations a “new” image was installed on the 500 school computers… Unfortunately, we teachers do not have any rights for installing or updating software, so this will be an “interesting” start…
But I did not know https://www.openprocessing.org/, so at least I got some new input out of it 
Thanks for that.