IMO, the easiest way to create what you want is to make a bookmarklet to hide all fixed elements. It won't run automatically on every page, but you can click on it whenever something's annoying you.
I used Ben Alman's jQuery bookmarklet generator with a filter from this existing Stack Overflow question.
$('*').filter(function() {
return $(this).css("position") === 'fixed';
}).hide();
I minified the code, generated the bookmarklet, then dragged the bookmarklet to my toolbar and, bam, instant tool.
If you really want this to run automatically on every page, you could check out TamperMonkey and use the same script.