Firefox won't recognize FF account username on a new install on new computer.

Question

I know I have the right username, email, and password, because I am looking at them on my existing laptop. But on installing FF 28 on my new laptop and trying to start the Sync process, I keep getting the message "unknown account."
I am getting this sign in message when I choose start sync process on my new computer. But I can't sign in, let alone start the pairing process.
I have done the pairing process before, recently, with my desktop and my existing laptop, so I know it works. Something about FF is not allowing it to recognize my correct email.
I'm on Win 8.1.
Thank you.

Answer 1

Thanks. It turns out I got my new computer at the exact time that FF dramatically changed their sync system. There was never a previous email or username or password, so the sign in page was misleading. Many users reading this will have to create an account, upgrade all their synced computers, unsync them, then sign in to their new FF accounts. Syncing will happen automatically.
The problem with "automatic" syncing was illustrated by my experience. Instead of syncing on the computer I wanted to copy, it synced from the computer I wanted to change. So, all my computers got all the bad passwords and duplicates of many good ones. It took a long time to straighten that out. But I made the changes on one computer and that changed the others. And new changes will automatically update. So it's an "easier" system. Still, I wish there was a way to choose when to sync. But thanks for the help anyway.