"YouTube Cookies Trouble" in Firefox 15 on Mac

Starting yesterday when using Flash Video Downloader in Firefox 15.0.1 running on a 2011 MacBook Pro running OS X 10.7.4 I started getting the following dialog box when I attempt to download any video from YouTube.

Youtube Cookie Trouble To continue download from youtube you have to clear youtube cookies. Do you want to clear Youtube cookies right now?

When I click the button for Flash Video Downloader in the menu bar I get this dialog box immediately and clearing cookies makes no difference, the problem persists and I can't begin a download. If I use the button at the lower left of the browser I can get Flash Video Downloader to put up the full window but all of the videos appear to be N/A, and don't show their size. If I click the Download button in that window nothing happens and in the Firefox download window the file is 0 kb in size.

This only happens on YouTube now and not other sites I frequent. It also behaves exactly the same on older Macs running older versions of Firefox and older versions of OS X. I have searched for the last day and can't find one single reference to this dialog box or this problem. Has anybody else seen this problem firsthand?

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