Extremely helpful!
I have generally switched off browser.display.use_document_fonts
(=0) in my browser so that I can see my own fonts on the websites. Unfortunately, there are some websites that absolutely want to use their own fonts to display symbols. This addon does exactly and perfectly (!) what I need: Switching browser.display.use_document_fonts via a new small button. Great! Many thanks for this!

Works well to toggle the display aspect of disabling external custom/web fonts (like several other extensions). But it is falsely advertised as being a privacy feature. This won't stop the fonts from being downloaded and so, won't improve your privacy in the slightest...

Works great and eliminates need to edit registry settings.