One of the most useful firefox addons I have used, and one which I use frequently when designing and developing websites is FireShot. It is especially useful when creating screenshots and measuring existing website dimensions when doing re-designs.
The main functions available with the free version of the program enable users to capture a temporary image version of a website and copy to clipboard, save as an image, or even email and print the output.
To use the website dimensions tool, install the addon into your Firefox browser. Then click the “Edit Capture” button which appears in the lower right hand corner of the browser window. This opens up an external editor which can be used to simply drag out a box and measure various parts of your website, or another website without having to use an external editor like Adobe Photoshop.
FireShot is also available for Internet Explorer.