I recently moved to a new computer, and have been rebuilding my desktop environment. I just wanted to list my Firefox plugins for future reference.
- The Web Developer Toolbar lets you view css, disable javascript, resize your browser precisely, outline block elements and so much more.
 - Colorzilla is invaluable for a developer like me to copy a color palate correctly.
 - S3Fox lets you manage Amazon S3 buckets and their content.
 - Firebug lets you examine web pages in detail, including network operations/
 - YSlow is an add on for Firebug that evaluates site performance based on a variety of metrics
 - Some kind of whois tool–sometimes you need to know who registered that domain name
 
These are my drop dead extensions for typical web development work. What am I missing? Do you have any to recommend?

I can’t believe I forgot to mention Live HTTP Headers: https://addons.mozilla.org/en-US/firefox/addon/3829/
That tool is so useful when you’re looking at caching issues.