How to fix: Firefox uses incorrect fonts on all webpages, regardless of their CSS

If you encounter a Firefox that does not care about your font settings but always uses specific fonts, you can fix that. Usually this should not happen, as it's not a default setting:

1. Open up the Preferences.
2. Switch to "Content".
3. In the "Fonts & Colors" section, click the "Advanced..." button.
4. Tick "Allow pages to choose their own fonts, instead of my selection above".
5. Confirm by pressing "OK".