My initial question was "What determines if a page can or can't be displayed in Reader View in Firefox?" but I found the answer on a Webmasters StackExchange question.
As far as I understand, sites that don't automatically show the option to enable Reader View have a problem with their code/tag/html/something. The Firefox support page for Reader View suggests something similar:
If a page is available in Reader View, the Reader View icon will appear in the address bar.
My question now is, if Reader View icon doesn't appear, is there a way to force enable Reader View on such pages?
I read this Reddit discussion but it doesn't answer my question.
I considered posting this question in Software Recommendations but I'm not sure an addon will provide a solution. Maybe this can be done with a script or a Firefox setting in about:config
or something else entirely?