Firefox 4.0 is a major release with an extensive list of new features. Highlights include:

• A new user interface:
  • Tabs are now on top and cycling through them is easier.
  • The home button has moved. 
  • There’s a new bookmark button.
  • You’ll notice a new tab grouping feature known as Panorama.

• Speed improvements. Version 4.0 is up to six times faster than its predecessor. Page load times and start up are both faster.
• New crash protection when a crash occurs in Adobe Flash, Apple Quicktime, or Microsoft Silverlight plug-ins.
• A revamped add-on system, which allows installation and browsing to occur in a designated browser window, instead of a pop-up menu.
• Improvements to performance, memory usage, and stability.

For a comprehensive list of the changes, see these release notes.