Thankfully Firefox has the "Allow pages to choose their own fonts, instead of your selections above" setting which you can uncheck so that Firefox always uses y

Thankfully Firefox has the "Allow pages to choose their own fonts, instead of your selections above" setting which you can uncheck so that Firefox always uses your preferred fonts. All pages can do is choose between serif, sans-serif, and monospace. And you can set a minimum font size, so that text is never too small no matter what the page does.