The people who answer questions here, for the most part, are other Firefox users volunteering their time (like me), not Mozilla employees or Firefox developers.

If you want to leave feedback for Firefox developers, you can go to the Firefox Help menu and select Submit Feedback... or use this link. (You'll need to be on the latest version of Firefox to submit feedback). Your feedback gets collected at http://input.mozilla.org/, where a team of people read it and gather data about the most common issues.

Not sure how above applies, but Im not sure about your problem.

That attribute is used to make a pinned tab glow when the title has changed.

• chrome://browser/skin/browser.css

There is also an unread attribute to show when a page has been reloaded and not visited yet, but no attribute for a change of the favicon. So it isn't possible to do what you want.
You would have to do a file compare to be sure that it is a different file.

Why would you want to check for a changed favicon?

My first answer applies because leaving this question as Firefox feedback with help the development team work this into the next version of Firefox.