In order to change the font that website uses, click on the menu button which is represented by three horizontal lines located at the top right corner of Firefox. Select "General," once you're in the page, search for "Fonts" in the search box located in the page, you can refer to the screenshot attached below as reference.

An option under the name of "Fonts and Colors" should appear as part of the search result, you can then change the default font, font size, font color, and many more. For a more detailed explanation, you can refer this Firefox article. https://support.mozilla.org/en-US/kb/change-fonts-and-colors-websites-use

Note that you lose special icon fonts if you disable website fonts (you get little boxes with the hex code).

• Settings -> General: Fonts & Colors -> Advanced
  [X] "Allow pages to choose their own fonts, instead of my selections above"