This is not the first post where you will find about useful firefox add-ons. But I will describe about the addons that I use most frequently and what I think about them.

  1. Firebug: Since I have a keen interest in developing web application therefore I like to play with CSS, HTML, JavaScript. During development I mostly get encountered with errors :( and there comes the firebug. It allows the debugging, editing, and monitoring of any website's CSS; HTML, JavaScript and provides other Web development tools. It also has a JavaScript console for logging errors and watching values, as well as a "Net" feature which monitors the amount of time in milliseconds it takes to execute scripts and load images on the page. 
    2. <li><strong><a href="https://addons.mozilla.org/en-US/firefox/addon/1865/">Adblock Plus</a></strong>:

    Its one of the most useful addons for firefox. It gets rid of all the unnecessary advertisements. It comes with some predefines set of filters to choose from. In addition to this, you can choose to block flash and other content that are unnecessary just with one click. It quickens the loading of webpages by blocking the unwanted content and loading the useful content first. This is a must have addon for everyone.

    <li><strong><a href="https://addons.mozilla.org/en-US/firefox/addon/2464">Foxy Proxy</a></strong>:

    A must have addon for those who work behind multiple proxies. It replaces the default firefox proxy manager with its feature rich manager. You can define multiple proxies, switch proxies with a click. You can even define a set of rules for different pages. You can create such rules on the fly with FoxyProxy. 

    <li><strong><a href="https://addons.mozilla.org/en-US/firefox/addon/13494/">Multilinks</a></strong>:

    Open multiple links in one click. This is the addon that I once used very frequently. No need to open several search results one at a time. Select all of them at once and open in multiple tabs using this addon. 

    <li><strong><a href="https://addons.mozilla.org/en-US/firefox/addon/9825/">Fastest Fox</a></strong>:

    Browse faster with fastest fox. It automatically recognizes multiple pages in a website and loads the next page automatically as you reach the end of the current page. It also provides parallelized downloads, dictionary, advanced searching, search suggestions and lots more. One of the addons that will certainly increase your productivity especially is you use search engines a lot.

    <li><strong><a href="https://addons.mozilla.org/en-US/firefox/addon/125440/">Facebook Photozoom</strong></a>:

    One addon that can be useful for Facebok addicts. Zoom photos with mouseover. No need to open a separate tab for seeing the full photo. See it then and there using this addon. 

    </ol>

    Here, I have provided a short list of the many useful firefox addons that I use. I hope that you find them useful too. I will try to post about more addons soon. In the meantime, feel free to comment about other addons that you use extensively.