Why does the local domain on MAMPA open differently in different browsers?

Question

Apache HTTP Server

Created a local domain.dev domain in MAMPA, for example.
For some reason, Firefox tries to open domain.dev/index.html/ (yes, with a slash at the end) and, of course, cannot display it.
Chrome opens the page as it should.
Opera does not display anything at all, just a blank screen.
Safari opens as needed.
Website without back-end, bare HTML/CSS/JS. OS X El Capitan.