This post summarizes the highlights of “What’s new for Web Developers in Firefox 7” by Christopher Blizzard for Mozilla Hacks:

• text-overflow: ellipsis. If text is too long to fit into a given box, this CSS setting cuts it off with a nice triple dot.
• WebSockets are back: with an updated (more secure) version of the protocol. They are now enabled by default on Firefox for Mobile.
• Canvas speed improvements.
• Restrictions on a site’s ability to resize the current window:
  • A site can only resize a window or a tab if it was created by window.open().
  • A site cannot resize a tab it it’s in a window with more than one tab.
  That’s a good one. Apps that resize a window with several tabs in it are a major nuisance. It’s just incompetent programming: Instead of opening a new window and then resizing it, just give it the right dimensions from the start. The above rules should put an end to this kind of shenanigans.
• Support for the Navigation Web Timing spec.

• Firefox 6: new features for developers

From http://www.2ality.com/2011/09/firefox7-dev.html