I can't type special characters on my Mac firmware password request
Finally and with the help of my friend Víctor I have managed to solve the problem. An important detail is that I use a Spanish keyboard layout. When you configure the firmware password from the recovery mode of the operative, your local keyboard configuration is loaded. However, when you try to enter the firmware password the distribution that is loaded is the American.

So, to enter the special characters of your password you will need to know how they're transcribed using your keyboard as an American keyboard. E.g.: in a Spanish keyboard, the # is written using ⌥ option + 3 and in an American, ⇧ shift + 3. In the login password screen of the firmware you'll have to use ⇧ shift + 3 to input it.

In honor of the truth, when you configure your firmware password a comment urges you only to use letters, numbers and punctuation symbols. I must admit that I didn't notice that warning. And being an advanced and very careful user, I established a potentially dangerous password for the integrity of my system without perceive it. I consider it's an important error in the system.

I can only reiterate the gratitude to my friend Víctor for his help this morning. We use his Mac to transcribe my password with an American keyboard layout.