Want to know specific details about an image you are looking at? You can quickly find an image’s width and height with the Image Spider add-on for Firefox.
Now I can not see where this would be useful for your normal day to day Firefox users. I can see where it could serve as an important tool for web site designers. For any image on a website you can pull up details such as the width, height, natural width, natural height, and HTML elements such as the alt, title tag and border tags.