It does not work on Firefox 14.0.1 with Firebug 1.10.0, on Windows Vista 32bits. When I click YSlow, it just shows blank box. When I click Run YSlow, nothing happen, and close when I click Close. It just only works to close. The old version (both YSlow and FF, I don't know exactly which version) is working well in the past. I noticed that problem since last week when my FF and YSlow automatically updated. Now, I'm using Google Page Speed plugin and it works very fine. Sorry guys, you should fix this asap. I like YSlow and I would like to continue to use it anymore.