Well, that's a new one. Maybe you have some code wrong in the header. If you change the "how to edit your header & footer" option to plain text, does it make a difference? (that option ins in your screenshot).
So I did a complete uninstall of Firefox and then installed it fresh. And the problem is solved!
Among with some other issues with other websites.
The downside is that I do not know what part of Firefox was causing the problem. Sorry for that.