Test what exactly? If your test is "The page renders how I expect when using the browser's default styling" then that's a terrible test, because that can change

Test what exactly? If your test is "The page renders how I expect when using the browser's default styling" then that's a terrible test, because that can change at any time without your control. You just have to accept what that is. There isn't a valid test for it. The fix for this is to define your own H1 margin and font size and then to test that the site looks correct with those values. Your test is should be that your styling works, not that Firefox's styling works. That'd be like testing a dependency. You shouldn't be doing that.