Handbook section 6.2 Browsers contains a few sections about how to setup Java and other plugins in various browsers.
Firefox and Chromium (among others) no longer support NPAPI plugins like Java, so these instructions are no longer valid.
I think you can still use the icedtea-web package to run downloaded Java Web Start applications manually, which should be explained in this section.
A commit references this bug:
Date: Mon Jan 1 10:07:34 UTC 2018
New revision: 51357
handbook: remove instructions for NPAPI plugins
- Neither Firefox nor Chrome support NPAPI plugins anymore and as such
the instructions to install them are not applicable.
- Tell users of opera to install the plugin directly rather than
redirect to Firefox's instructions. I have not validated that this
still works. Google shows mixed information for Opera's NPAPI support.
- While here remove an extraneous word 'new'
Reported by: Randy Westlund <firstname.lastname@example.org>