Firefox is a great browser, and even better, when it comes to plug-ins or extensions. I use countless extensions in a day-to-day basis, so I'll mention the ones that I like the most for development, or personal use. Please comment and recommend others, because I'm sure that there are many out there that are great and I'm missing out.

Live HTTP Headers (sniff http traffic, great for AJAX)
I use this extension every single day, it monitors every single http packet and it's extremely easy to use. I can see banner calls, JavaScript loading, Ajax calls, you name it. It is not as complete as Ethereal, but it's much simpler and therefore more useful for an everyday basis.
http://livehttpheaders.mozdev.org/

Web Developer (most tools web developers need)
The name is self-explanatory, it has every single utility that a web developer needs. Web Developer just released version 1.0, after a full year of development. I will not even name everything because it does a lot. The functions I use everyday are viewing CSS, JavaScript, and response headers, but again, there will be something in there for you; I guaranty it.
http://chrispederick.com/work/webdeveloper/

RoboForm (fill up forms, save passwords)
I use this to store my unlimited passwords for hundreds of sites I use. It is also extremely useful when you're developing forms and need to test out data… or even if you're just browsing and this one site requires you to fill up a bunch of unnecessary fields, just click one button and RoboForm will do it for you. It also has a built-in password generator that I use sometimes, and protected notepads… I only wish there was a way to synchronize my profile at home and at work.
http://www.roboform.com/

FasterFox (improve performance)
FasterFox is awesome. It allows you to customize network performance, although I just set it to max and leave it alone. The other reason that I love this extension is completely unrelated: you can right click the task icon and clear your cache. Although this sounds silly, I found that Firefox always caches your SWFs and there is no other way to refresh than clearing your cache. If you're a Flash Developer, or have to deal with some, this fact alone is worth installing it.
http://fasterfox.mozdev.org/

Console2 (better JavaScript / CSS Console)
Firefox 1.5 modified a little the JavaScript console, yes, now it's the JavaScript / CSS console, and keeps throwing errors that you do not really need to see. It may be useful for someone, but it is a pain for me. Console2 made a good job by allowing you to filter your errors and only display the ones relevant to you.
http://forums.mozillazine.org/viewtopic.php?t=318102

Sage (nice RSS reader)
I am now actually using SharpReader, but until not too long ago, Sage was one of the best and easiest to use RSS readers that I could find. It is already built-in my browser, so I don't have to open any other software, and I really like the way it displays my feeds.
http://sage.mozdev.org/

Customize Google
It adds a bunch of little features, such as the Google suggest, which is working great… I don't know why it's still on Beta. It also adds direct links to Google Images, and remove banners from your gmail.
http://www.customizegoogle.com/

foXpose (Mac-like page split)
I barely use this, but I still think it's great. This extension adds a little icon to the bottom-left corner of your browser, and allows you to see all your tabs simultaneously in a Mac-style view.
http://viamatic.com/firefox/

Session Saver (save current tabs, cookies)
So if you're a web developer, saving your cookies may be a bad idea… but this extension doesn't force you to do it, it allows you to. What I like the most of this, is that if FF ever crashes, or say Windows crashes, you can open FF and all your tabs will still be there.
http://adblock.ethereal.net/alchemy.cgi/SessionSaver

Page Saver (save image of full length web page)
If you need to send screen shots, print-screen will only save the viewable area, and saving to a pdf may not include all the elements of the page… this extension allows you to save exactly what you see to a PNG. When you install it, it is set to save 25% of the viewable resolution by default, which I found crappy. I recommend changing it to 100%, and if you need to scale it down you always can.
http://pearlcrescent.com/products/pagesaver/

How can you tell if a site is being viewed mostly by a developers' community? While the normal site has 90%+ I.E. visitors, my site has the following stats:

Google Desktop: 10.8% Views; 39.7% Visits
Firefox: 46.1% Views; 20.9% Visits
Microsoft Internet Explorer 6.x: Views 27.3%; 9.6% visits

Which means that most of the people added my RSS feed to their Google Desktop that pulls my feed infinite times a day … followed by the typical developer that uses Firefox, and last but not least, the business man that is searching for Web 2.0 and AJAX information that uses I.E.

It is amazing how stats change drastically from one site to another… I have other sites where I.E. is 97%, and some others that are viewed mostly by Ad Agencies where Safari reaches almost 40%.

That is exactly why you need to QA for every single browser out there!

I’m a proud user of FireFox 1.5

Firefox 1.5 came out today, and I didn’t think it twice before upgrading. I use plenty of extensions and most of the ones I had worked.

Two of the ones didn’t work are: Roboform, and Live HTTP headers.

I downloaded the latest Roboform extension from http://www.roboform.com/browsers.html and it got it working immediately.

Live HTTP headers is a must and I use it everyday. I also found the latest extension at http://livehttpheaders.mozdev.org/ but site is acting extremely slow and I haven't been able to download it.

 So there’s this huge ticket regarding the Firefox+Roboform problem. Seems like it’s not Firefox, but the roboform adaptor that actually uses some features it shouldn’t be using that were discontinued in version 1.0.5 for security reasons. Anyways, I just downloaded 1.0.6 and of course it broke. But! I upgraded the adaptor and it works. I just can’t live w/o by Roboform.

For those who use firefox and roboform,  do not upgrade to the latest version (1.0.5) because it will immedialy crash. I just had to installed 1.0.4 back.