Firefox sync unable to sign in

I have set up a new Windows 11 PC (brand new installation). I installed Firefox and tried to sync, because I want to copy settings, history etc. from my Windows 10 PC. However, it fails. I enter my password, then it asks me to enter a code sent to my email. I enter that code, and it says "Bad request" and "Something went wrong. Please close this tab and try again." It's not the most helpful error message. All I can do is retry, but after 20+ attempts, I think there is something wrong with the 2FA. I'm able to successfully log in to https://accounts.firefox.com/ on the browser without any issue. That uses a 2FA code from the authenticator app on my phone. Any ideas? Is there a better way of copying my browser to the new computer? I was thinking of just copying the Firefox profile folder, but I don't want to end up with it duplicated since my Windows 10 PC is still active.