It would be a lot better if it worked for individual tabs only

Does not work with 64-bit Firefox 56.0.2 on Windows 7 64-bit operating system. No button visible.

Worked with Firefox 68.0 but not with Waterfox Classic 56.2.12.

You might like to add a strict_min_version to the extension, and update the AMO page.

The toolbar button is ambiguous; nothing more than the word on (or off), with an icon that does not match the icon here.

I prefer Enforce Browser Fonts:

https://addons.mozilla.org/addon/enforce-browser-fonts/

– its toolbar button is more meaningful.