Setting the firmware password is optional, so you, or some other user of your Mac set that firmware password.
When did you first notice that your Mac started asking for the firmware password when you wanted to change the boot drive by booting with the Option key?
Finally, do you still get the firmware password screen, if you boot without holding the Option key? (Your Mac should boot to the default boot drive?) and, that would give you your "normal sign-on screen".
BTW, if you go to an Apple store to get your firmware password cleared, you will actually need that proof of purchase, such as your sales invoice. Apple is pretty picky about what they accept for proof of ownership