• Better performance on startup and shutdown, as well as for JavaScript.
  
• Protection from out-of-date plugins has been implemented.
  
• HTML5 video can be played in full screen.
  
• Resizeable text areas in web forms.
  
• A new Troubleshooting Information page (about:support) is available.
  
• MailNews return receipts (MDN) now use notification bars.
  
• History expiration has been reworked, and form field autocomplete now also expires its value and orders them based on usage.
  
• On Linux, desktop notifications (e.g. mail alerts, finished downloads) show up in the native environment through libnotify.
  
• The SSL security system has been changed to fix a renegotiation flaw.
  
• CSS :visited selectors have been changed to block ways that websites can quickly check a user's browsing history.
  
• The WOFF format for downloadable website fonts is now supported.
  
• SMIL animation in SVG is supported, and SVG attributes mapped to CSS properties can now be animated using that. For even more dynamic display, CSS transitions are now supported as well.
  
• On Mac OS X, plugins supporting Core Animation rendering can now draw faster and more efficiently.
  
• New CSS attributes such as gradients, background sizing, and pointer   events have been implemented.
  
• A new HTML5 parser is used for all HTML documents, and new DOM and HTML5 features including the Drag & Drop API and the File API, which allow for more interactive web pages, are now supported.
  
• JS-ctypes support has been improved, allowing in-application JavaScript code to access native system libraries.
  
• Support for WebGL and for Direct2D acceleration on Windows has been added (all disabled by default).
  
• Mac OS X 10.4 "Tiger" support has been dropped due to newer features needing system interface only available on 10.5 and higher.