ScribeFire Blog Editor
A full-featured blog editor that integrates with your browser and lets you easily post to your blog. This is the be-all and end-all of addons.. I think this is the greatest invention of the century.. am I over-reacting? I have over 40 domains, it's difficult to keep up with them all.. I still don't.. but with this tool, I really have no excuse not to.
It lets you load in all your blogs (even if they are Drupal blogs.. wordpress.. blogger.. etc.. etc) and you can modify blog posts on-the-fly.. including loading up previous blog entries.. choosing the tags, categories, and technorati tags and trackback urls.. it has fully-featured html integration, as well as "instant" add you-tube video, add flickr image, etc. 
I'm so happy with this addon.. I'm beside myself with glee at the amount of time-saving it's already saved me having only had it for a couple of weeks!

Littlefox for Firefox  
(more of a theme than an add-on)
LittleFox, designed for optimal screen usage, leaving lots of room for browsing. For Firefox 2.0 and 3.0.
It turns all your toolbars into tiny icons.. which is sweet.. so you can fit more toolbars in.. It takes a little getting used to, but once you do, it's so much better than having all these toolbars taking up all the space.

SnagIt Firefox Extension
Captures images, scrolling web pages and more right from Firefox!

Ohh no wonder I love this extension.. I think I must've bought it lol.. Oh yeah I did, cos I needed to once send a "fax" via email as a screenshot so I filled out the form and captured it to PDF. But that's not why I love this extension.. it's so useful for taking quick images.. even for this blog post, where I have a little "screenie" for all the firefox addons.. I just click a button.. select the area I want to capture.. it opens in snagit.. from there, I can add drop-shadows, adjust sizing, highlight areas, add some fancy borders, and then I use the

Flickr output addon to send it straight to Flickr which copies the html code to my clipboard so I can immediately insert the image and not have to worry about saving the image to my computer, and uploading via FTP, and creating my own html.. it saves so much time.. and I'm addicted to it, and think it's an absolute must-have. There's so much more you can do with it.. like capture from any document, website, video, pdf file, send directly to email or IM + Skype, or FTP, etc.. it's crazy all the things that they've already thought of. There's so much I have yet to explore and work out with it, but if nothing else, the capture from webpage to flickr is worth it! Every image on this blog post was captured with SnagIt and edited and sent to Flickr, so if you scroll over these images, you should have a fair idea of the versatility.. including some of them which kept the "links" of the images hotspots.  (- Update.. 1st Oct 2008 - since Flickr completely deleted my account within hours of going Pro.. this is definitely something I will never do again.. and if you have an account with Flickr.. suggest you backup your Flickr often.. there are free websites that will enable you to do this easily.. I friggin hate Flickr now)

You can download a free trial of SnagIt here.

extEMM
Must-have tool.. it automatically saves all the tabs you have open so you can "restart" Firefox. You don't know how much I rely on this when I open too many things and just need to close Firefox to get some memory back on my poor lil lappy lol, it's not an essential tool until you actually use it even once... then it becomes your best friend!

PDF Download

PDF Download by Nitro PDF Software helps you regain control over PDF files in Firefox. When you click on a PDF file, PDF Download lets you know, can tell you how big it is, and can then give you the choice to open, download, or convert it to HTML. PDF Download helps to:

* Stop PDF files crashing your browser
* Stop PDF files taking forever to open
* Make PDF files download like other files in Firefox

Download Statusbar
View and manage downloads from a tidy statusbar - without the annoying download window popping up!

Shareaholic
Found something awesome online and can't wait to pass it on? Easily share, bookmark and e-mail webpages with Shareaholic.

MeasureIt
Draw out a ruler to get the pixel width and height of any elements on a webpage. More info

FireShot
FireShot is a Firefox extension that creates screenshots of web pages. More info

I don't like how it really wants you to upgrade to Pro all the time, but for the usefulness of it's freebie screencapture of whatever webpage you are on or whatever "region" in the webpage you are on is just so convenient.. especially when it can automatically upload the image to your FTP and copy the link to your clipboard. If it automatically copied the embed code and allowed you to capture it to Flickr, it would be even better... in the meantime, it can "email" it to flickr but that still doesn't copy the html code.

ColorfulTabs
Colorful Tabs colours every tab in a different colour and makes them easy to distinguish.
I use it to differentiate the sites I'm on.. like.. facebook tabs all have the same colour, etc etc.

Copy as HTML Link
Creates an HTML link to the current page using the selected text and copies it.

Copy Link Name
Copies the name (source anchor) of a link.