Some Firefox add-ons I’ve found indispensable for web development and everyday use:
Cookie Manager Button – Creates an icon on the navigation toolbar for quick access the Cookie Manager.
Add-on page: https://addons.mozilla.org/en-US/firefox/addon/3402
QuickJava – Allows quick enable and disable of Java and Javascript from statusbar.
Site: http://quickjavaplugin.blogspot.com/
Add-on page: https://addons.mozilla.org/en-US/firefox/addon/1237
Firebug – Edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
Site: http://www.getfirebug.com/
Add-on page: https://addons.mozilla.org/en-US/firefox/addon/1843
Flashblock – Enables/disables flash for web sites that you designate.
Site: https://addons.mozilla.org/en-US/firefox/addon/433
Add-on page: http://flashblock.mozdev.org/
WebMail Notifier
Site: http://webmailnotifier.mozdev.org/
Add-on page: https://addons.mozilla.org/en-US/firefox/addon/4490
Tabs Open Relative
Site: http://jomel.me.uk/software/firefox/tabsopenrelative/
Add-on page: https://addons.mozilla.org/en-US/firefox/addon/1956
User Agent Switcher – Adds a menu and a toolbar button to switch the user agent of the browser. (Useful for testing browser-specific sites.)
Site: http://chrispederick.com/work/user-agent-switcher/
Add-on page: https://addons.mozilla.org/en-US/firefox/addon/59
CSSViewer – A simple CSS property viewer.
Site: http://www.nicolashuon.info/?page=work&type=projects&id=cssviewer
Add-on page: https://addons.mozilla.org/en-US/firefox/addon/2104
Firecookie – An extension for Firebug that makes possible to view and manage cookies in your browser.
Site: http://www.softwareishard.com/blog/firecookie/
Add-on page: https://addons.mozilla.org/en-US/firefox/addon/6683
CacheViewer – GUI Front-end for “about:cache”.
Add-on page: https://addons.mozilla.org/en-US/firefox/addon/2489
IE Tab – View web page displayed in IE but within Firefox tab.
Site: http://ietab.mozdev.org/
Add-on page: https://addons.mozilla.org/en-US/firefox/addon/1419
Stephen Sulzberger's Blog 