I am running Firefox Developer Edition 67.0b4 and did not have this issue. I noticed that you are using Firefox 65. Maybe updating to Firefox 66.0.1 (current version) would resolve this?
I'm guessing it is actually a https thing. Do you have that on? Because I am currently on a FF66 (different computer) and having the same error. But given the error it looks more to be because there is no https version of the site.
Which I'm fine with. If you don't have https I don't need to go to your site.
More of a principle thing. SSL is easy to enable and the whole web should be encrypted. I fully support all the major browsers' decision to warn users about sites that aren't https (even without login). It doesn't feel like there's a good excuse to not have https enabled.