This list is by no way comprehensive (or new), but does list a few extensions that I found irreplaceable that you could use with your firefox browser.

1. Firebug

The first in the list is Firebug. Written by Joe Hewitt, Firebug has the ability of cutting down your coding time. You’ll no longer find yourself using the Error Console which is inbuilt in Firefox, Firebug relays all the bugs that it can find, even on pages beyond the one you’re looking at (via Ajax). You can customize your firebug to work with only a few sites, you can preview the site and change the CSS on the fly. Of course that’s just scratching the surface.

2. MeasureIt

As a web designer, you must have had issues with certain images or layouts being off by just one pixel. In comes MeasureIt, a tool to measure the height and width of certain segments of the page.

3. ColorZilla

ColorZilla is a cool extension which allows you to check out colors using a color picker, or you can choose from an entire palette.

4. Screen Grab

If you don’t want to use a tool like Webshot to take screenshots of entire pages online, you could use Screengrab! a nifty tool that will allow you to take screenshots of what is currently on your screen - you can even choose the portion of the screen you want to “grab”. This is especially useful if you want to take a shot of a personalized page which requires a log in.

5. Web Developer

Firefox Web Developer is a favorite for many, many designers and coders I know, I personally don’t use it as much as I should, but do believe that this is a very useful tool in the long run, especially once you get used to it. It adds a toolbar to the top with a ton of features.

6. IE Tab

The tough parts of web development is the compatability issues developers face with Internet Explorer. IE Tab extension allows developers to switch between IE and Firefox browsers with just one click, especially really helpful with viewing the results of your CSS and javascript codes.

7. HTML Validator

Another aspect of a perfect design is getting your pages validated according W3C standards and Html Validator is the perfect tool for it. This plugin validates your pages within your local system and dosen’t send your pages somewhere for validation. It also upgrades your view of the page source along with and error log and an example on how to fix the error.

8. CSS Viewer

CSSViewer is a tiny plugin that instantly displays the current CSS attributes of a html element when you roll over it on the page itself. With its ease of use and the information it displays most of the designers will just fall in love with it once they get used to the plugin.

9. Fire FTP

FireFTP converts your Firefox into a simple and powerful FTP client where you can upload and download your files from remote servers. It also allows you to manage multiple ftp accounts within a single Firefox profile.