• The new Data Manager is opened instead of the previous separate management windows.
  
• Page zoom is being remembered on a site-specific basis now.
  
• WebSockets have been disabled for security reasons.
  
• Lightweight themes (Personas) support for the Address Book, Composer, and MailNews windows.
  
• Deferred Session Restore.
  
• The browser tab bar is now scrollable (also using the mouse wheel) to cope with tab overflow. Includes an "All Tabs" navigation button to quickly list all open tabs.
  
• News messages can now be deleted from local storage.
  
• Website search fields now feature a "Add a Keyword for this Search" context menu entry.
  
• In some places where previously a notification bar appeared (like remembering log-in passwords or after installing an extension), SeaMonkey now shows a "doorhanger" below the location bar.
  
• SeaMonkey now uses Omnijar which results in fewer installed files, less fragmentation and better startup time.
  
• Sync (f.k.a. Weave) is now included directly in SeaMonkey. Sync allows you to keep your browsing history, passwords, bookmarks, preferences and tabs in sync across different devices (computers) in a secure way.
  
• Mac changes: PPC support was dropped, 64-bit support for 10.6 "Snow Leopard" was added.
  
• The new DoNotTrack HTTP header can be configured in Preferences, Privacy & Security
  
• WebGL and D3D are now enabled where supported (hardware acceleration, see about:support)
  
• Improved OpenType support
  
• HTML5 Forms API support
  
• Various Add-on Manager improvements
  
• Faster bookmarking and startup performance (due to Places changes)
  
• Flash on Mac OS X improvements
  
• Memory usage improvements