Script Summary: Why does YouTube not let us see where we've been? This script is a "must have" to see which videos you have already viewed. Highlight previously visited video links on YouTube. 8/23/10
Updated 8/23/10
It's a frustration that nobody needs: You click on a video only to find that you have already seen this one, but youtube does not highlight the previously visited links to let you know that. We don't know for sure why youtube has chosen to hide this essential information from us, but this script is a must have for making YouTube.com tell you which videos have been recently viewed - as is the standard default behavior for visited links on web pages.
What the script does:
1. On most areas of the youtube site, including "Related Videos", "More From", any of the Browse screens and the search results screen, the script causes visited video links to show up with a red background or as red text in order to differentiate from the default blue used by youtube for both visited and non-visited videos.
2. In addition, on the search results screens only, if youtube labels your video as "Previously viewed" (must be signed in for this), then the entire video block will be colored with a gray background.
3. For further emphasis, the youtube favicon 
TROUBLESHOOTING:
If the script doesn't work, there is probably a conflict with another script or style you are using with youtube.com. Try disabling them and it should work fine. (Please report problems here instead of posting bad reviews, etc.) The script also requires that [Options->Content->Colors->Allow web pages to set their own colors] is in its default state of [checked].
SEARCH RESULTS PAGE ONLY:
YouTube may label a video as "previously viewed" even when your Firefox browser history has expired that link. There are also times when YouTube drops its "previously viewed" label for some unknown reason, but these will still show up with 1 and 3 above based on your browser history behavior. In other words: the grayed background and highlighted links will not always coincide, especially if you log in on multiple computers. You must be signed in to allow youtube to track your "previously viewed" videos. Otherwise, the script will base its highlighting entirely on your browser history.
