I have a "Kill Sticky" bookmarklet to do that job, as the first thing on my bookmarks toolbar. I'm clicking that thing all the time, it works great. So long stupid sliding navbars, enter-your-email popups, fixed video autoplayers, etc. I think I got it from here: https://alisdair.mcdiarmid.org/kill-sticky-headers/

I use this bookmarklet: javascript:(function()%7B(function%20()%20%7Bvar%20i%2C%20elements%20%3D%20document.querySelectorAll('body%20*')%3Bfor%20(i%20%3D%200%3B%20i%20%3C%20elements.length%3B%20i%2B%2B)%20%7Bif%20(getComputedStyle(elements%5Bi%5D).position%20%3D%3D%3D%20'fixed')%20%7Belements%5Bi%5D.parentNode.removeChild(elements%5Bi%5D)%3B%7D%7D%7D)()%7D)()

You could use ublock origin (or similar) to "block" the offending elements.