Gmail is shitty about supporting non-Chrome browsers. Earlier, they shipped this animated loading thing that literally took 15 seconds on Firefox. That was the thing that finally got me to leave and go to Fastmail.
Just now, I had to log into an old Gmail account to set up forwarding. I got this misleading error message.
Then, I tried the same thing in Chrome, and it worked. I have no sympathy for a multibillion dollar company breaking basic functionality on browsers that they do not own. I don't think they created this bug intentionally, but I do think it was intentional to not do basic testing on non-Chrome browsers.