There are plenty of addons for web developers (both for IE and FF browsers).
Here, I am introducing most popular toolbar for this purpose.

Alexa Toolbar

What if you could take all the websites in the world and sort them by popularity?
Alexa has done just that. Click on the Top Sites tab to unlock a whole world of information about websites. You can see a list of the most popular sites in the world, based on global rankings something only Alexa can provide you. Popular sites by country is another Alexa exclusive. For example, if you want to know what sites people in a
specified country like to visit, you can find out.

Perhaps the most powerful Top Sites feature is located in the "By Category" tab, where you can see lists of top sites in thousands of different categories.
Say that you are interested in "extrasolar planets;" Alexa has a category for that: Science > Astronomy > Extrasolar Planets. There you'll find a list of 33 different sites about extrasolar planets all sorted by popularity.
Top Sites by Category is a great way to discover new sites.

Toolbar is necessary because google search results show alexa ranking of each result as well.
Hence, you can know in advance about a site before visiting it.
Remember, sites having less than three month age might have very low ranking because ranking shows the
three months performance.
Also ranking is performed on the basis of visitors having alexa toolbar installed.

Web Developer
Adds a menu and a toolbar with various web developer tools.

Firebug
Firebug integrates with Firefox to put a wealth of web development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.

FirePHP
FirePHP enables you to log to your Firebug Console using a simple PHP method call.
All data is sent via response headers and will not interfere with the content on your page.
FirePHP is ideally suited for AJAX development where clean JSON and XML responses are required.

Fire Shot
Press PrintScreen and can't grab the entire webpage because it is longer than the browser window? You need FireShot!
It adds the ability to take a screenshot of the page, edit it and save, copy to clipboard, send to external editor or e-mail it.

DOM Inspector
The DOM Inspector (also known as DOMi) is a developer tool used to inspect, browse, and edit the Document Object Model of documents - usually web pages or XUL windows. The DOM hierarchy can be navigated using a two-paned window that allows for a variety of different views on the document and all nodes within.

Color Zilla

With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. You can Zoom the page you are viewing and measure distances between any two points on the page. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes. DOM spying features allow getting various information about DOM elements quickly and easily. And there's more...

Measure It
Draw out a ruler to get the pixel width and height of any elements on a webpage.

Pixel Perfect
Pixel Perfect is a Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML.

SQLite Manager
Manage all your sqlite databases using this lightweight extension. This extension will be useful to developers who want to create and play with sqlite databases.

Internet Explorer Tab
Enables you to use the embedded IE engine within Mozilla/Firefox.
This is a great tool for web developers, since you can easily see how your web page displayed in IE with just one click and then switch back to Firefox.

For other informative stuff visit eziHub.com