This is because the site has the child theme of Extinct theme loaded but does not have the child theme style.css file modified to remove the custom styles included there for loading custom font Weston. If you unload the child theme and instead load the parent theme Extinct, things should look the same in all browsers. Firefox does not take the child theme style.css settings into effect since it is still referring to remote font downloads on our servers due to security reasons.
If you plan to use one of the fonts listed in the theme options Fonts tab, pls either empty the child theme style.css file or load the parent theme itself.