I have mistakenly locked my MacBook Air and now it’s requesting for a firmware password which I don’t know

As detailed in this Support Article, you will need to bring your Mac along with original purchase receipt to Apple.

Set a firmware password on your Mac - Apple Support

When setting a firmware password, it is up to you to remember this password and it is not saved by Apple or any place else. This is a high level of security that locks your computer with only information that you know without a way for anyone to bypass/reset it.

In case it wasn't entirely clear from the post above, if you don't have the original receipt for the machine from Apple or an authorized retailer, and you can't remember the firmware passcode, it is now a boat anchor. Apple won't be able to help you.