I think it is very informative to share some work habits and Internet habits with others. I have found that sometimes the most productive thing one can do is watch my fellow developers or other people work for 30 minutes. We all have different styles. Even when people are using the same browser, or the same tools at work, different people will know different techniques.
In that spirit of shared knowledge then, I offer my short list of Firefox browser addons - not the 'coolest' but the ones I use every day. There are only a couple of generally useful ones, the rest are software development addons. But if you use a website, maybe people have written add-ons for that site (e.g. FaceBook Photos, Twitter addons, LinkedIn Companion etc). Think about your most visited sites and see if they have a plugin.
Fox Marks While you may not want to trust your passwords, sharing your bookmarks among machines means you will never 'lose' an interesting article. I recommend setting up folders in your bookmark toolbar, so you don't run out of bookmark real estate. If you have alot of bookmarks across machines, setting up folders after you add all the machines is essential. Then, this bookmark synchronizer becomes powerful with some effort on your part. Have an "Interesting Stuff" folder, a "Finances" folder or whatever you like. Then, you should be able to find groups of bookmarks easily. Sure, you can tag bookmarks in Firefox 3 as well, then just type them in the location bar, but I like to have folders of bookmarks so I can open different sets of tabs depending on what I want to do when I get to the computer.
Ad Block Plus
This is great because you will not be exposed to advertising, but it's so good that you will not know you are missing out. It is subtle and effective.You simply subscribe to a service during the installation that will keep the list of 'advertisements' (presumably advertiseing network IPs and domains) to hide up to date automatically.
I work on the acquisitions side of a large website. For testing, we are forever needing a quick way to manage cookies for test cases, make sure our pages have an SEO consciousness and test out different ideas for marketing.
Web Developer Toolbar: The main features I find useful for this toolbar are to delete domain cookies for testing, and inline editing of css to see changes to the page dynamically during final development. If I were a full time web UI developer, I would find this tool to be at the core of what I would need, because the other features allow you to inspect and control the layout and contents of the page.
Firebug: I am sure you have all heard of and use Firebug. One thing I picked up recently was to use the 'Inspect' button to inspect a particular element on screen during debugging. It provides a great way of answering 'what is the name of this form parameter for the text field' for example. Then, if you want to edit the value inline for testing purposes, you can. I use this to submit errorneous values for drop downs and other form inputs during testing. The other advantage of this tool is to give you an understanding of page weight, using a pie-graph, and also allows you to really see and understand the web requests a page makes, and the cookies that are set. The 'Net' tab is a great visualization of the traffic your page creates.
YSlow : The performance team at Yahoo really captured plenty of useful information in this plugin. I use it for understanding the next few things I wuld do to make my site run faster from the real user experience point of view. Many of its recommendations are generic and not applicable to startup companies (who may not want to use a CDN). But the important thing is this will let you have some metric about how your site is performing, such that you can see the improvement. This is especially important when selling refactoring projects to management - such things should be done in conjunction with performance improvement work (such as gzip-ing your web traffic via Apache's gzip compression technology), then if your refacotring to a better architecture has latency or page-rendering implications, yu have softened the blow.
What other Firefox addons have you tried and found to be useful?