The red border-colour appears only when applying custom CSS to URL or Domain. Your feedback however has given great insight into how it actually can be mistaken for an error. This feature has since been removed in the new version; along with a bug-fix and some small visual changes.

It's also important to remember that the 'Mozzila.org' domain isn't affected by any custom CSS rules. This is because Mozilla explicitly doesn't let Firefox Add-ons alter the DOM on their domain. Make sure to test it out on another website!

Thank you for checking out my Add-on!

Regards,
Bryce