What does a firmware password protect?
Installing a firmware password will prevent anyone without the password from selecting a different boot disk to start up the computer. It will also disable some startup key sequences, like getting into single-user mode. I don't know how the firmware password interacts with something like rEFIt.

However, if someone can open your computer, they can disable the firmware password by pulling one of the sticks of RAM and/or just access your hard drive directly and do what they want with it.