Instructions, as requested Rated 5 out of 5 stars
1. Install the addon, if you don't have it already installed.
2. Find the file 'clickYoutubeDownloader.jar'.
a) On Windows, it will usually be in C:\Documents and Settings\<your-username>\Application Data\Mozilla\Firefox\Profiles\<some-random-string>.default\extensions\YoutubeDownloader@PeterOlayev.com\chrome\ . (Thanks MasterEd000, I don't use Windows so I couldn't check.) Alternatively, you can use Windows' "Find" utility and look for the file.
b) On Linux, it will usually be in /home/<your-username>/.mozilla/firefox/<some-random-string>.default/extensions/YoutubeDownloader@PeterOlayev.com/chrome/ .
3. Replace it with this file: http://inf.ufrgs.br/~vbuaraujo/foo/clickYoutubeDownloader.jar
4. That's it!
@rysss and others:
If the addon's author does not appear, I might make a new addon based on this one and publish it when I have some time. I guess I will do it until January 15 if an official fix is not published. If I do, I will announce it here. Thanks for the feedback!