There are many extensions in Firefox are really useful for web development and web designing, here listing of most useful Firefox Add-ons.
The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. It is designed for Firefox, Flock and Seamonkey, and will run on any platform that these browsers support including Windows, Mac OS X and Linux.
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon in the status bar when browsing.
YSlow analyzes web pages and tells you why they’re slow based on Yahoo’s rules for high performance web sites.
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.
Lets you load pages in IE with a single right-click, or mark certain sites to *always* load in IE, It is great tool for diffrent browser check.
Manage any SQLite database on your computer.
FirePHP enables you to print to your Firebug Console using a simple PHP function call.You must have Firebug installed and the “Net” panel enabled to use this extension.
Seoquake is a Mozilla Firefox extension aimed primarily at helping web masters who deal with search engine optimization and internet promotion of web sites. Seoquake allows to obtain and investigate many important SEO parameters of the internet project under study on the fly, save them for future work, compare them with the results, obtained for other, competitive, projects.
The Browser Window Resizer is useful for testing different screen sizes. It accurately resizes your browser so you can test to see what a web page looks like in all of the standard resolution sizes.
Supports the 640×480, 800×600, 1024×768, 1280×800, 1280×1024, 1600×1200 resolutions.
Resize the current page via the Context menu, Tool menu or Toolbar button.