More details are available on the release notes page.
HTML 5 improvements
Here's a look at Firefox's HTML 5 support since Firefox 16.
|HTML5 test results|
Not exactly leaps and bounds of improvements, but there's steady improvements being made. The biggest jump was in Firefox 17.
|Firefox Kraken benchmark|
|Firefox running Google's Octane|
When you take a look at Octane, Firefox 19 has regressed quite a bit from the massive gains in Firefox 18.
|Firefox doing progressively worse in SunSpider|