Below are the Firefox plugins I generally use and recommend to people:

• AutoCopy
• ColorfulTabs
• Download Statusbar
• DownThemAll!
• external IP
• Firefox Showcase
• Flagfox
• Xmarks
• Groowe Search Toolbar
• LinkedIn Companion
• New Tab Homepage
• Session Manager
• ShowIP
• Speed Dial
• Tab Mix Plus
• Video DownloadHelper
• WebMail Notifier

This is a list of web dev tools that I use, am I missing anything?:

MS Windows editors:

• Notepad++, some tips can be found here.

Online resources:

• CSS Cleanup Tool
• HTML Cleanup Tool
• Blueprint CSS Framework (e.g. Example)

Firefox development plugins:

• Add N Edit Cookies
• DNSStuff Toolbar
• Netcraft Toolbar
• EditCSS
• Firebug
• Html Validator
• LinkChecker
• Server Switcher
• Total Validator
• View Cookies
• Web Developer
• YSlow
• IE Tab
• Page Speed from Google
• HttpFox, to view GET and POST data from HTTP requests
• Fiddler, to view GET and POST data from HTTP requests

Online development communities:

• Stackoverflow.com
• Serverfault.com
• Superuser.com

SEO Tools:

• Free SEO report
• The Top 15 Free SEO Tools

I will be updating this list from time to time.