Your battery may require re-calibrating after the firmware update, but it does sound more like something is making your CPU spike, which would account for the drastically reduced battery life and the fans spinning.
To fully re-calibrate the battery, do the following:
- Run the MacBook on the battery until it goes to sleep, then allow the machine to fully run down (wait for the front light to stop glowing), which can take 4 - 5 hours.
- When this is done, remove the battery and hold down the power button for at least 5 seconds
- replace the battery, and reconnect to the mains to charge the battery
At this point you can start the machine up again and check, it should say 0% charge and give the capacity as close to 100%. Really, if it's a month old and already showing 90%, you might want to return it and ask for a new battery.
As for the firmware and spinning fans, I doubt what I told you will correct it, but you could also try resetting the PR RAM, done by starting your MacBook by holding Command (apple-key) Option, P and R whilst holding down the power button. It should chime twice when reset.
If all else fails, take it to a genius. Apple staff get snippy about battery complaints, but stick to your guns because a 10% drop in one month could mean an essentially useless battery within 6 months.