If you are a web site developer you probably have got inspiration from other site sometimes. Again, while developing a new project, you had to deeply and thoroughly test your creation. These are the main reasons why - if you use Firefox or Chrome - you should consider installing the Web Developer extension.
As an example, using the "Disable" drop down menu, you can disable:
4) Meta redirect;
5) fonts minimum size;
6) page colours;
7) popup block;
Those disabling command will help you isolate problems or errors.
Another interesting set of tools is available in the CSS menu. I must admit, I use it a lot to check newly created pages. The best feature is the Border Box Model: with it you can select any part of the web page and see the associated CSS style.
Debugging the code behind a page is very important. The Web Developer toolbar provides a set of additional tools that is very handy. You can validate your CSS style, your feeds, html code, links and so on. Interesting enough, you can do it with a live page or locally. Infact you can check CSS stylesheets and html code from local files - and, believe me, that is incredibly useful.
I understand that writing the full list of features for the Web Developer extension, would be crazy. While I'm writing this very article, checking the toolbar, I realise I would need more than just one article. Therefore I ask you once again to trust me: install the extension and see for yourself. You won't regret it.