Alt-ctrl-r is also a key command that is supposed to work in firefox, but it never has, for me. Instead, pressing f9 does the trick. I think I tested it once so far in Waterfox and, indeed, f9 does not seem to do anything. I will try accessing the reader through the menu or the alt-ctrl keystroke and see.

BTW, as nice as the reader function is, everybody should be aware that it’s not perfect at determiningg what is “useful content” on a page, and you may occasionally miss something important while using it. It is nice for reading articles, just be aware that it’s not 100% dependable.