Prepending `about:reader?url=` works for me, but only for pages that Firefox shows the button on anyway. For pages where it doesn't show a button, prepending `about:reader?url=` says "Failed to load article from page", which is to be expected because...

>Sometimes the button just doesn't appear (why not, reader mode usually works fine).

... the Reader Mode script uses heuristics to identify if the document has an article. So it can't work on a document where it can't find an article to extract.