Here's a full overview of what's new in Firefox 17:
NEW: First revision of the Social API and support for Facebook Messenger NEW: Click-to-play blocklisting implemented to prevent vulnerable plugin versions from running without the user's permission (see blog post) CHANGED: Updated Awesome Bar experience with larger icons CHANGED: Mac OS X 10.5 is no longer supported DEVELOPER: JavaScript Maps and Sets are now iterable DEVELOPER: SVG FillPaint and StrokePaint implemented DEVELOPER: Improvements that make the Web Console, Debugger and Developer Toolbar faster and easier to use DEVELOPER: New Markup panel in the Page Inspector allows easy editing of the DOM HTML5: Sandbox attribute for iframes implemented, enabling increased security FIXED: Over twenty performance improvements, including fixes around the New Tab page FIXED: Pointer lock doesn't work in web apps (769150) FIXED: Page scrolling on sites with fixed headers (780345)