__.: Web Development :.__
* Web Developer – Huge suite of tools for designing and debugging.
* ColorZilla – Eyedropper tool that plops hex codes into the clipboard.
* Session Manager – Particularly useful when developing applications that use session variables.
* Total Validator – This will send the code output from your local machine or intranet server to the various W3C validators.
* IE NetRenderer – For Linux kids who don’t trust wine for IE testing. (Windows kids will find IE View Lite to be useful)

__.: Opera-isms :.__
* Fission – Turns the address bar into a progress bar while loading.
* Extended Statusbar – Gives total page size and loading speed statistics like Opera.
* All-in-One Sidebar – A clone of Opera’s sidebar.
* Fast Dial – A clone of Opera’s speed dial.

__.: Greasemonkey Scripts :.__
* Google Image Relinker – Brings you straight from the search results to the image.
* Remove All Facebook Ads – Pretty much does what it says.
* Facebook Chat Killer – Another obvious one.
* Google Account Multi-Login – Turns the sign out link into a drop-down select box for multiple Google accounts.
* Myspace Custom Style Remover – The only way I’ll ever use MySpace!
* Disable Text Ads – Works everywhere but Gmail.

I’m actually using the default theme at the moment. I’m waiting for some themes to catch up to the Firefox 3 platform.

+ Greasemonkey with these scripts:
– Flickr Buddy Icon Reply
– Del.icio.us+x (changes the style of the site)

+ Adblock Plus with these filters:
Easylist+Easy Element
Myspace Junk Filters
ABP Tracking Filter

+ Delicious Bookmarks

What theme are you currently using for firefox? I pretty much covet these themes from GrApple.