Customizing the font family used in Firefox reader mode is helpful to increase online reading experience. This can be achieved according to the following procedure.

  1. Open the address about:support in Firefox and open the profile folder. Open Firefox user profile folder from about:support

    Fig. Open Firefox user profile folder from about:support

  2. If the folder chrome does not exist, create it. Then create a CSS file userContent.css in the chrome folder with the following contents as example, where I’ve adopted my favorite font KaiTi for reading Chinese articles and Noto Serif for English contents. 
    .moz-reader-content { 
  font-family: "KaiTi", "Noto Serif", "Times New Roman", serif;
}
  3. Open about:config in Firefox and change the option toolkit.legacyUserProfileCustomizations.stylesheets to true.
  4. Restart Firefox to take effect.

If you want, more styles and features can be customized with the help of the Developer Tool in Firefox.