@Roman - Sorry your system doesn’t offer recovery services (intel systems only)
The proper way is to use the grey bootable CD the system came with and using the menu option to reset your firmware password with it.
There is another way and thats removing one of the SO-DIMM’s and PRAM battery and then with no cord plugged in pressing and holding the power button for 15 secs to fully drain the logic board. Now you should be able to get to your drive.
You might encounter this error ***"To continue booting, type ‘mac-boot‘ and press return”***
Follow this Apple TN: [https://support.apple.com/en-us/TS1812|PowerPC: "To continue booting, type ‘mac-boot‘ and press return" Message]