browserSettings.useDocumentFonts

A BrowserSetting object whose underlying value is a boolean.

By default, of course, web pages can specify the fonts they want using CSS properties like font-family. This setting enables an extension to instruct Firefox to ignore the fonts specified by the page, and only use system fonts.

Its underlying value is a boolean:

• true: use the fonts specified by the web page. This is the default.
• false:  use the system fonts.

Set the setting to false:

function logResult(result) {
  console.log(`Setting was modified: ${result}`);
}

browser.browserSettings.useDocumentFonts.set({value: false}).
  then(logResult);