Google's rainbow G icon displays as my favicon for all YouTube pages and videos that I bookmark. But in the tab, YouTube's red rectangle with a white play arrow still appears.

This issue first appeared in my bookmarks several weeks ago. I do not have this issue in my Chrome browser. I've already tried the Forget About This Site option in History, but to no avail. I've also tried clearing cached web content and site data in Firefox's Options, again to no avail. Thanks in advance for any help.