From the link:

If someone gets into this discussion, I recommend that you simply comment out all the GNOME styles so that this does not happen again in the future.
Just comment that line in your userChrome.css file
import “firefox-gnome-theme/userChrome.css”;
You can found it here:
/home/YOUR_LINUX_USER_NAME/.mozilla/firefox/YOUR_PROFILE/chrome/userChrome.css

Seems to be userChrome.css doing that. Which would explain why I have never seen it but I do want to do custom CSS so this may pop up for me too.