How do I set a default font?
How do I change the default font and font size in FireFox?
Alle svar (1)
https://support.mozilla.org/en-US/kb/change-fonts-and-colors-websites-use If wanting to use a Extension https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer/ also possible : https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/
Please let us know if this solved your issue or if need further assistance.