The Firefox Web Developer Extension is the best tool I've adopted in 2007. It makes testing a websites functionality and interface a breeze. Here are my nine favorite features - all available in one click!
- View the CSS definitions that are effecting any element on the page.
- Outline every block on the page to quickly identify the important structures.
- Quickly resize the window to see what it looks like at different screen resolutions.
- See what cookies the site is setting.
- Disable javascript and see how your site looks to a search engine, older browsers, most cell phone users, etc.
- Disable external images. What would your site look like if the image server went down?
- Display all "alt" tags. (Perfect for SEO!)
- Disable a stylesheet or edit one in real-time.
- Check the page in multiple popular validation services.
