That's definitely a KeyChain issue. Claiming an account works for me with the newest version on Firefox. Creating claimed accounts does still not work, but I want to support KC for that too, therefore it is now activated by default. I reported the issue here on GitHub last year already, but it seems as if they didn't solve it yet...

By deactivating the option Use KeyChain For Signing in Settings -> Security you can prevent using KC (may require a page reload). If they won't fix it soon, I will add a confirmation dialog/popup, so that the users can choose which signing method to use there.