Web Developer – Add a Web Developer toolbar of various web development tools with web developer extension. This extension add a toolbar with various web development tools like css, cookies, images, forms.
IE Tab – Designers and developers can view their CSS projects on Internet Explorer using this extension.
W3C Offline Page validator – The web developers nightmare – Validating. Offline Page Validator is a simple tool that will send your page to the W3C validator by simply clicking an option on the right click menu, works both online and offline.
Palette Grabber – Creates a color palette for Photoshop, Paint Shop Pro, GIMP, Flash, Fireworks, or OS X based on the current page.
FireFTP – FireFTP is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.
ColorZilla – Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies. With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes.
LiveHTTPHeaders – View HTTP headers of a page and while browsing in real time.
MinimizeToTray – Minimizes Mozilla windows into the system tray. Also includes a right clickable tray icon menu to further load only the windows you want visible.
FasterFox – Fasterfox allows you to tweak many network and rendering settings such as simultaneous connections, pipelining, cache, DNS cache, and initial paint delay.