Just to narrow things down a bit.
I had the same problem and discovered the following. When using Firefox, my battery ran out of time faster, my mac was hot and fan ran periodically. When using FF with Silverlight, battery time was even worse, fan almost always on and computer was hot.
On 3rd test, I moved all my open pages from FF to Safari and have been testing the battery time with FF closed and Safari open. So far, my fan has not come on once. Going to watch a video online to see if it's better as well. My diagnostics show Silverlight, by far, is a power hog, FF was the next biggest problem but not bad.
Basic details from rough testing.
6/21/12 with Silverlight running in FF
1.34 min at 12.39 pm full power on screen, full charge
.32 min at 1.39 pm
just over 1 hr.
6/22/12 - activity monitor open, FF, no silverlight.
3.47 min at 9.38 am 3/4 power on screen, full charge, silverlight off and on.
.19 min at 12.28 pm
Almost 3 hrs.
6/21/12 after recharge. Full charge, 3/4 power on screen lighting. All work in Safari, FF closed.
4.39 min at 5.55 pm using safari and not firefox.
3.59 at 7.02 pm
waiting to see.
Hope this helps anyone.
Using a MBP 15", late 2011, with Snow Leopard, v. 10.6.8, Hybrid hard drive
I had the exact same problem with my macbook pro (Purchased June 2012).
My fan was running constantly, and I couldn't figure out why. I simply saw in my activity monitor that "iTunes" was using 61% of my CPU! I force-quit the application and the fan immediately began to slow down.
Thanks for the help!