Recently a change happened with Firefox on Android which stopped me from using the work around of adding something to my reading list and opening it from there to force a page into reader mode. With that in mind I tried to find and then finally make a bookmarklet to force a page into reader mode for me.
window.location = 'about:reader?url=' + window.location;
Access to 'about:reader?url=...' from script denied
Here is a "work around" that might ease your particular pain:
It will prompt you with a url you can copy and then paste into the URL bar.