Basic plugins that anyone may like:

• Gmail Manager – check and manage multiple Gmail accounts
• History block – prevent some sites from crowding your web browser history
• Echofon - Twitter client (formally known as TwitterFox)
  • I’m looking for a better Firefox Twitter client, please comment if you know of one.
• PDF Download – Decide whether you rather download or view PDF
• Tabs Open Relative - Open new tabs to the right of your current tab

Plugins specific to web development:

• Abduction! – Snag a screen shot of web pages
• Chrome View – Open current page in Google Chrome web browser
• Clear Cache Button – Clear browser cache with a simple button next to your refresh button
• Colorzilla - Select colors from within web pages
• DNS Cache – turn on/off DNS caching in Firefox
• ElasticFox - Amazon EC2 client
• Firebug - Javascript debugger
• Firesizer – resize your window to specific sizes
• HTML Validator – check web pages are well formed
• Live HTTP Headers - monitor HTTP headers found in web pages
• MeasureIt - Measure anything in a web page
• Open in IE – open current page in IE
• OperaView – open current page in Opera
• Regular Expression Tester – Test regular expressions from within your browser
• RESTClient – generate and test REST formatted HTTP requests
• S3Fox – Amazon S3 client
• Safari view Win – open current page in Safari
• Server Spy – display the server type in the status bar
• ShowIP – display the server ip address in the status bar
• User Agent Switcher – switch to other user agents for web site testing