Here's my second post in this "toolkit/workflow" series, featuring my favourite Firefox add-ons.
Firecookie: Handy add on to Firebug to help you manage site cookies, be it exploring, deleting or creating on the fly.
Drupal for Firebug: great tool that helps you tear apart what Drupal is doing under the hood with its content and themeing. Some people prefer Devel, but I think the processing overhead for that module sometimes isn't worth it, and Drupal for Firebug gives me all I need.
PageSpeed: Another Firebug add-on from Google that helps grade your site's speed and offers suggestions on how it can be improved. Great for debugging your css and looking for improvements.
Web Dev Bar: A pretty solid web development assistance tool, although I find myself using it less and less these days. Firebug is my main tool.
HttpFox: A solid http traffic watcher. Load it up and start browsing to see just what pages and assets the websites you're visiting are actually loading. I think Fiddler has more options and features, but you need to set up .NET and configure it and blah, blah, blah. In a pinch (and most other times), HttpFox will give you all that you need.
PixelZoomer: You ever wanted to have photoshop open while you were testing a page in Firefox, and be able to measure the actual pixels of your layout, or look closely at that 1px border and make sure it was actually 1px? PixelZoomer will take a screenshot and present it in browser allow with zoom and measuring tools so you can take a closer look at the design.
Screengrab: An excellent screen capture add-on. You can take snapshots of the browser window, an entire site, or use the selection tool to pick out a particular piece. Unfortunately it doesn't do a good job of screen capping flash elements, but for everthing else it's good.
LastTab & Too Many Tabs: I'm a tab freak. I often have dozens and dozens of tabs open at a time. I'm not so much a multi-tasker as I am keeping tabs open on things that I'm researching or learning or need for future reference, but aren't important enough to bookmark. LastTab helps me navigate back and forth between two tabs by using the Ctrl-Tab shortcut key (normally this keyboard shortcut rotates one by one through all your open tabs), and Too Many Tabs allows me to stash excessive tabs away into another portion of my toolbar for later use.
Memory Restart: Of course, with all those tabs open, memory usage can become a problem. This add-on watches how much RAM Firefox is eating up and will warn you when it gets too high. You can restart the browser manually very quickly by hitting the button in the interface, or you can configure it to auto-restart after the memory usage climbs above a certain threshold.
Xmarks: This is a great bookmarking synching service that I've been using since before Firefox brought out their own synching service. I'm quite fond of it, and just haven't moved off to the Firefox native version. The best part is that you can synch not just across devices, but across browsers as well, since there are versions for Chrome, IE and Safari. Your mileage may vary.
Gleebox: This add-on opens up a bunch of keyboard shortcuts to help users navigation the web using only the keyboard. It's kind of like Quicksilver for your browser. I debated adding this one to the list as I'm not really using it anymore - I've started to master Firefox's default keyboard shortcuts - but it's still worth a look if you hate your mouse.
Speed Dial: An innovation first created by Opera (one of many) that has been adopted by Firefox via an add-on. Allows you to tie keyboard shortcuts to your favourite sites as well as provide an interface in blank windows/tabs for getting to those faves quickly.
RSS Ticker: There's a lot of news out on the web, and if you've ever dived into RSS feeds, you'll quickly find that you can't keep up on it all. I use Byline (below) for my main feeds, but for less important news and feeds, I've been using RSS Ticker to "passively" keep an eye on those feeds. Productivity gurus beware: the scrolling can be distracting. I wish there was an easier way to disable it and turn it back on again.