(title omitted due to this add-on being beyond words) Rated 5 out of 5 stars
Hard to review X-Ray while, at the same time, trying not to sound like a crazed sychophant. I'll try to let pictures do most of the talking.
I had a web page, I broke it, I turned on X-Ray, now my grandmother could figure up what is wrong with it. See the screen snapshot at http://www.smithline.net/images/Firefox_X-Ray_add-on.png. Notice how the border on the right doesn't line up the way the border on the left does. X-Ray shows exactly which DIV tag is incorrect. This was my first time using X-Ray (or is it "taking an X-Ray"?) and I suspect it just saved me 20-90 minutes. If the developer's home page has a contribute button I'm going to throw him a few bucks just for my saved time.
Note: my lo-res capture made some letters hard to read. This was done to keep the image size small. The original looks fine.