Firefox Reader Mode has some heuristic where it will not be available on certain pages. One way to improve readability in Firefox for all websites by default is to use the Language and Appearance section in Preferences to display all web pages using your chosen fonts and colors: Click on Advanced, select the fonts and font sizes you want, and unclick "Allow pages to choose their own fonts, instead of your selections above." Then click on "Colors," select the colors you want, and under "Override the colors specified by the page with your selections above" choose "Always."

What I do with this is to set everything to inverse video (white-on-black) and small monospace fonts. Basically makes Firefox look like w3m.