Firefox 52 ESR

1. So you'd like to run Java Applets?
2. Mozilla tells you that Firefox 52 ESR is the latest version supporting it
3. You actually managed to find the download directory at https://archive.mozilla.org/pub/firefox/releases/52.9.0esr/

You install the 52 version - start it and it immediately updates it self to a version not supporting applets! See e.g.:

• https://support.mozilla.org/de/questions/1157765