I thought it would be useful for posterity to document which add-ons to Firefox I'm currently using.

Bookmark Duplicate Detector

Does what it's name says - keeps your bookmarks free of redundant entries.

ColorZilla

Let's you find out what colour something is on the current web page.

DownThemAll

Download manager (I'm using 1.0b2) which makes big files downloading a lot faster. Not 100% compatible with all sites, so you do need to resort to the built-in manager sometimes.

Firebug

The best web page debugger and inspector.

Foxmarks Bookmark Synchronizer

I use this to synchronise my bookmarks between work and home.

IE View

I don't use this as much as I once did, but there are some pages out there that unfortunately only render in IE.

One of those nice productivity enhancers - it modifies the cursor when you hover over a link to indicate what the target file type is. Notice in the image below how it tells me that this is a link to a Word Document.

Example of link alert extension

Live Writerfox

Let's me launch LiveWriter to blog about the current web page.

View Trace.axd

Roger's fantastic add-on that makes debugging ASP.NET applications so much easier.

Web Developer

The original web developer toolbar. Firebug supercedes some (but not all) functionality so I don't use this as much as I used to.

YSlow

Requires Firebug and gives you a "performance report card" of the current web page.

I haven't mentioned DOM Inspector or Talkback, as they both come with Firefox.