Other cool Firefox add-ons for web development (and more):
Adds a menu and a toolbar with various web developer tools.
User Agent Switcher (Web Developer required)
Adds a menu and a toolbar button to switch the user agent of the browser.
Lets you load pages in IE with a single right-click, or mark certain sites to *always* load in IE.
Open a page in Google Chrome on Windows.
Open pages in Opera from Firefox context menus.
Safari View Win
Adds a context menu entry to open the currently viewed web page in Safari on Windows.