Here is how I tried to exit the recovery mode on an iPhone 3GS, new bootrom, using the POWER + VOLUME DOWN + HOME buttons. As you can see, the phone restarts by itself in the same mode. Previously, it had a jailbroken iOS 6.1.2 with ultrasn0w on it, and baseband 5.13.04. However, signal was really low.
This is the first step in my attempt to upgrade to the iPad baseband 6.15, then re-downgrade it again to 5.13.04.