No doubt, Firefox is the most widely used browser among the designers and developers. One of the many compelling reasons why Mozilla Firefox is so popular is it's endless trove of free awesome add-ons. These amazing add-ons have been a great help to the designers and developers. Here are the top 5 essential Firefox add-ons that designers should know and should be equipped with!

Firebug

Firebug is the one of the most popular Firefox add-ons among the designers. It allows you to edit, debug and monitor the CSS, HTML and JavaScript of any live web page. The best of all, every change that you make can be previewed live and instantly.

• Html tab helps you inspect elements and check their CSS attributes. It allows you to add/edit/hide/delete the HTML of the CSS.
• Layout tab illustrates all the offsets, margins, borders, padding and sizes for you.
• Net tab shows you when the file started and stopped loading relative to all the other files. It also filters the list by type for you to find out the total size and download time for a particular type of file.

Web Developer Toolbar

Web Developer Toolbar is also one of the most useful add-ons for the designers. It adds a menu and toolbar to the browser with a tons of essential tools that allow you to code quality websites and troubleshoot problems easily.

• Ruler measures the width and height of any element in the website.
• Line guides horizontal and vertical guides for you to check if the alignment of the website is correct. 
• Resize window displays your website in different screen dimensions.
• View image information displays all the details of the images that you have created and used in your website at at one glance.
• Display element information shows the attributes, position, font, ancestors and children of each element.

IE Tab

IE tab is an extension from Taiwan that allows you to check your website in IE without leaving Firefox! IE tab opens up your file exactly like it would look like in the version of IE that you have installed in a convenient tab. Right click then click on the Open in IE Tab link and that's it!

ColorZilla

ColorZilla is an online eyedropper, it assists you with color related tasks. You can pick and choose colors of multiple pixels in a page and save them in a custom palette for later use.

• Webpage DOM Color Analyzer analyzes DOM element colors on any web page and create a corresponding palette.
• Zoom allows you to zoom into any details of the page to pick the exact color to use.
• Copy color code after you have picked the color and paste it into another program. You can choose to copy the color code in few formats.
• Statusbar displays the color code that you pick by just mouse over on any pixel in the website. You can also change the color format to be displayed at the statusbar.

Seo Quake

Seoquake is a Firefox SEO extension. It is an extremely useful tool for web masters specially who deal with search engine optimization(SEO) and internet promotion of web sites. It is a good tool to estimate effectiveness, competitive ability of their SEO efforts and identifies important parameters that are related to the search engine.optimization.

• SeoBar is an additional toolbar which shows the values of selected parameters for the page currently opened in the browser window.
• Parameters can be changed by checking which parameters to be displayed. Sea Quake also gives users the opportunity to create your own parameters, group them into presets and switch between these presets.