No, I'm talking about a new firefox account that I am trying to create on a different computer in the school where I work. I enter my email, pw and firefox tell me "I sent a verification code to your email addr. Enter the code here". The problem is that the email with the code arrives about 20 minutes later and the code is no longer valid.

The timeout for the code is actually 20 minutes, so you can still give it a try as you are rather close.