Wšě wotmołwy (1)
You don't need to provide a birthday to create a Firefox account, or to install and use Firefox on your devices.
Do you want to change or add your birthday for your Google account? You might have used your Google account to sign up for a Firefox account.
If that's right, you should use Google's support pages: https://support.google.com/accounts/
If that's wrong, can you tell me what you're trying to do?
Wot Kaylee G. změnjeny