What the Browser provides is what it can only do anything else beyond that is custom fonts that are installed by another software or you have to use something like userChrome.css to make those modifications. The only other thing that can change fonts is the O/S itself.

You can only select a font-family that way. You would have to use CSS rules to select a specific variant. For the user interface these rules would be in userChrome.css and for websites in userContent.css. *https://developer.mozilla.org/en-US/docs/Web/CSS/font *https://developer.mozilla.org/en-US/docs/Web/CSS/font-weight *http://kb.mozillazine.org/userChrome.css *http://kb.mozillazine.org/userContent.css ---- *https://www.userchrome.org/how-create-userchrome-css.html *https://www.userchrome.org/what-is-userchrome-css.html