I've seen similar problems in Firefox when sites have their css <link>s in <body> instead of <head>, which is what this site is doing. But this works when UA is spoofed to 53, so it might not be that.
Bug 1726368 Comment 5 Edit History
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I've seen similar problems in Firefox when sites have their css <link>s in <body> instead of <head>, which is what this site is doing. But since this works when UA is spoofed to 53, it might not be that.