I'm a huge Firefox fan, by the way. Here's a rundown of all the extensions I use at work for my day-to-day work. The reason for posting this is actually to be a reference for my co-workers, who never can remember the name of "that one cool extension Nate had".
- DOM Inspector - Install firefox in advanced mode to get this
- Adblock - Essential for any browsing. I don't block everything, just the annoying ones. Innocent and helpful ads (google) or ads on sites I like always get to stay.
- HTML Validator - Leave it off when you're not using it, as it can slow you down. Shows you live when pages validate and when you view source, why they don't.
- ColorZilla - Perfect for grabbing colors. SO nice to have when working on HTML & CSS.
- MeasureIT - Good for troubleshooting broken float and table designs. I wish it would remember boxes inbetween uses and allow changing box sizes after you draw it.
- IE Tab - See what you're looking at in IE without searching for the blue -e- shortcut that you've probably deleted. Caveat: won't log you in if you're in a password-protected area.
- BugMeNot - Skip logins for sites that you don't care about. New version automatically keeps trying until it finds a login that works.
- Tabbrowser Preferences - Not sure if I still need this since Firefox 1.5, but it tweaks the tabs to work just the way I like.
- Platypus - WYSIWYG editing of other people's sites. Delete and move parts of pages until the site works how you like. Fix sites with awful background colors. Then use Greasemonkey...
- Greasemonkey - Save scripts (like those generated by Platypus) to re-modify a site every time you visit it. Google for greasemonkey scripts to see what you can do.
- Web Developer Toolbar - The single most useful tool for any and every web developer. An obvious required must have. If you make web sites, you need this, period.
- Gmail Space - This XUL application lets you use your Gmail account to store and transfer files.
- FireBug - Great tool for inspecting and debugging HTML and javascript, and tracking AJAX requests.
- Live HTTP Headers - Lets you watch raw HTTP headers - very valuable for debugging things like caching, cookies and load balancers
- Google Notebook - I use it for sharing bookmarks between systems (I don't want to snyc bookmarks) and noting them, as well as jotting down some ideas when I want to finish the thought somewhere else.
- ErrorZilla - On HTTP errors, this gives you options to check the google cache, ping the server and so on.
- Reveal - Makes surfing and using tabs more visual. Also eats your memory.
- CSSViewer - When you turn it on, it attractively shows all the CSS properties for whatever you put your mouse over. Disabled because I hardly ever used it.
- ScreenGrab - Makes an image of your entire web page. Disabled because I don't use it very often.