Under Fonts, use the drop-down menus to select the font and font size of your choice. Close the Settings page. Any changes you've made will automatically be saved. Some websites have custom fonts; to disable them:
I need some help, I thought I had followed all the steps I had found online to create and install a userChrome.css file so I can change the font for the general user interface in Firefox but it doesn't appear to be loading for me.