Thanks for the stars, greatly appreciated for my first add-on!

Due to your comment I have added new features to help with hidden data dynamically inserted. From the release notes:
- You can now manually preform a scan on a page from the add-ons pop-up menu.
- Option for checking copied text for zero-width characters.
- Option to preform a scan on the page again when the tab becomes active.

Notes on why it might not work:
Content scripts (scripts injected) are disabled on some of the Mozilla sub-domains.

CSP (Content Security Policy) can also be used to block add-ons and other sources (common concern is XSS) from running. This however, can be disabled at your own risk if desired.

Have yet to see a site other than Mozilla that blocks the add-on.