From MozillaZine Knowledge Base

XUL

• Adding items to menus with overlays (talks about browser's context menu too)
• Adding an Apply button to dialogs

Javascript

Commonly used techniques

• XulPlanet has an article on preferences
• Printing debug information to JavaScript Console

XPCOM

• File IO
• Using observers for cross-window notifications
• Run other applications

<browser> related

• Using progress listeners
• Executing your code each time a page loads
• Enumerating tabbrowser tabs
• Opening a new browser window

Other

• Inserting text at cursor
• Using XMLHttpRequest

Techniques

• Differential Inheritance in JavaScript using __proto__