Problem update: unless someone saw this and did something to the account creation timeout, it appears the solution is "wait" - it's been about 2 hours since I first got the message, so hopefully others experiencing the problem will see it resolved soon.

I have this problem as well and have been waiting forever. Is there anyone at Firefox that knows anything about this? Or, why use Firefox Sync when there are third party products? (Like, why use Microsoft when there are alternatives?)

This has to be the worst managed transition I’ve come across. There was no warning in Firefox at all and if I hadn’t added a new computer to the house I wouldn’t have even known this was happening. I already have multiple Mozilla related accounts, but now your stupid system wants yet another one, but won’t let me create it. It was completely unclear whether you could log on with your existing Sync account or any other Mozilla account, but it you try that you get locked out. Once locked out you can’t even use “Forgot password” which is just demented.

I’ve been trying this for a couple of weeks now. Maybe it’s time to just accept that it’s broken?

You need to wait a few hours if you get the "Attempt limit exceeded" error message.

You may have to clear the cookies as well and make sure to allow cookies from the accounts.firefox.com domain.

After two days, I was able to create the account.

That's a major shoot-itself-in-the-foot for Firefox.

The whole point of the "improved" sync service, as widely reported, was: "Mozilla's main motivation to improve Sync is to make it easier for users to set the system up, as it believes that it is the main reason why Sync is not used by more users of the browser." (http://www.ghacks.net/2014/01/12/mozilla-plans-update-firefoxs-data-synchronization-feature-version-29/.)