See also this Twitter thread with test results. Firefox 52 and lower generally show the error message; Firefox 53 generally doesn’t.
A few testers reported seeing a message one time, but not after repeating the test. Others reported seeing the error message in a brief flash. I have seen the former myself, but not the latter.
In addition, it appears that this bug may occur on Firefox 52 on Ubuntu as well. In general Ubuntu test results are slightly different from Mac and Windows ones.