It appears Mozilla Firefox now completely rejects websites with weak certificates. As in a “Secure Connection Failed” error page, with no way to bypass the error.
For example, one of the largest hosts in the world, OVH, has an “F” score certificate and Firefox completely rejects customer logins to their dashboard. (I called their support twice and of course they said it was my problem.)
I’m using Firefox 59, which is nearly the latest version.
In other words, even if you have https setup with a “green lock” certificate, you still aren’t safe from major browsers shutting you down. This could be really bad for business–imagine losing 12% of your traffic (Firefox) without even realizing why.
Test the strength of your website’s certificate here: https://www.ssllabs.com/ssltest/analyze.html
If you have a low score, you might be completely blocked by Firefox. If a major technology company like OVH missed this, you can imagine a lot of other businesses will suffer too.