Localhost sometimes doesn't run

I have a strange problem with trying to get WebGL builds. It started in 2018.3.8, it still continues in 2018.4.0.

Usually, the game's loading bar doesn't proceed, the game never starts. When I close Firefox and get a new build, then even the loading bar doesn't appear. I get an "Unable to connect" error.

The picture shows when Firefox isn't closed after I get a build.

I try all these with an empty game project, by the way. I made sure the empty scene is added to the build, the game works when I click on index.html file.

Edit: This happens in Firefox. Opera works fine.