1. Flash SwitcherFlash Switcher is a must have for Flash Developers, it lets you choose between different versions of flash so you can be sure your code is compatible with all existing versions of Flash. View more info on Flash Switcher
View more info on Firebug
3. Live HTTP HeadersLive HTTP headers shows you the dialog that happens between the web server and your browser. Figure out why things aren’t redirecting, why search engines aren’t properly indexing your content, and what website is pushing you cookies in realtime.
View more info on Live HTTP Headers
4. FireShotEver want to send screenshots to clients? Or take an image off a page and use it in a mockup? Fireshot lets you take screenshots of entire pages, selected areas, or the current window and export it to an imaging program. Great for quick mockups, client meetings and portfolio pages.
View more info on FireShot
5. Tamper Data
This plugin allows you to modifty GET and POST parameters on any page. Use it to test form processing code, test for security issues and diagnose bugs. It’s only issue is that it won’t work in tandem with Google’s web accelerator. View more info on Tamper Data
6. Load Time Analyzer
See how long your web page takes to load for different speeds. Especially useful for developers whose audience may still be using dialup. View more info on Load Time Analyzer
Ever one what the exact dimensions are of a piece of web page? Need to create a screenshot? Simple and easy to use. View more info on MeasureIt