We don't have a Kindle Fire to test on, but if someone's got one and they could run this: http://www.scirra.com/labs/sbperf/ It's our favourite performance test to check out new devices. It will report a score after about a minute.
Here is a promissing results. I ran the benchmarks on my kindle fire 1st gen with the dolphin browser. And the dolphin jetpack add ons. It enhances html5 games. I got around 35 fps and a score of 36. Much better than the default browser.