Webdeveloper : This is a very good Firefox extension for web developers as well as bloggers. It adds a webdeveloper menu in Firefox with lots of options. You can disable cache, cookies, images, style sheets and JavaScript for any webpage just with a click of a button. You can also view the source of webpage, style sheet information and other hidden elements. There is a menu tab to validate your website XHTML and CSS with W3c Standards. You can highlight block level elements, tables and headings.

Firebug :Firebug is a similar Firefox extension like Webdeveloper but it provides some advanced functionality and editing techniques, inside the Firefox.  You can inspect all the web elements of a page in real-time just by clicking on that area and you can monitor your site’s performance while it is loading in your browser.

Colorzilla : With the help of Colorzilla Firefox extension, you can pick and get a color  reading from any point in Firefox page. You can copy and paste the hexadecimal code in another program. It has a built in color palette browser which allows choosing colors from predefined colors sets and you can save them as a custom palettes. You can zoom in the webpage and you can measure the distance b/w two points in page.

Measureit : Measureit is a Firefox extension which allows you to draw out a ruler on a webpage with the help of mouse and it will show the Width and Height of the drawn in pixels. This is very useful when you want to take an idea of your block level elements during theme and layout design.

Fireshot : FireShot helps in taking screen shots of web pages (entirely or just visible part) in Firefox. It provides some simple Image editing tools and allows you to insert text and graphical annotations in the screen shot. You can upload your screen shots to websites, you can send them to external editor or email and you can also save them locally in different image formats (PNG, GIF, JPEG, BMP).

Adblockplus : Adblock plus is an extension to block all the ads and banner available on any website those are taking too long to load the site, just right click on the ad and block that; it will not appear on the site. But I don’t recommend this to my blog readers

IE Tab :  As the name suggests, IE tab Firefox Extension allow you to embed Internet Explorer in tab of Mozilla. Web developers can test their site in Internet Explorer through Firefox itself. There are still so many sites which only run in IE, so by using this extension you can see those sites in Firefox.

SimilarWeb :  If you want to do a research on Websites, this extension is extremely helpful. When you open any website in Firefox, then in a Sidebar, it will show other websites that are similar to the current website. You can do a parallel search in different search engines using this SimilarWeb Firefox extension.

SurfCanyon : This Firefox Extension improves the search relevance of different search engines. It saves your time during search and automates the discovery of results from Google, Bing, Yahoo!, Lexis Web or Craigslist. You just search for your query using any of the search engines and when you click on the appealing search result, SurfCanyon re-ranks the result and brings forward more relevant results from other search result pages.

Zemanta : It extends the capability of your email client and blogging dashboard by providing the relevant content suggestions to the current text. It show relevant images, links, articles, tags in a very handy interface. You can reuse and link to the resultant content with just by a simple click. It also provide an option for copyright filter for resultant results.

Video DownloadHelper : This Firefox extension allows you to download the embedded video from different websites like YouTube, Metcalfe and others. It also works with the audio and Image galleries. You just surf any website, it automatically detects all the available media on that site and then you can download it just by a click. For example you are watching a Video on YouTube then it will detect the video file and you can start downloading that video to your local machine.

The above listed eleven Firefox extensions really upgrade my browsing and blogging experience with all the useful tools inside the browser itself. The great thing is they are available free with full functionality.