At least this should theoretically make it easier to block those horrible popup modals, if they're all grouped under one element:
dialog {display:none}In the meantime, NoScript[1] frequently avoids popups entirely by just showing the page contents with no JS at all, and Kill Sticky[2] cleans up the pages that require JS to show you the content you actually want. As a bonus, it also nukes those stupid sticky headers!
[1] NoScript for Firefox (incl. mobile) & Chrome-based desktop browsers: https://noscript.net/getit/
[2] Kill Sticky bookmarklet for all browsers including mobile: https://github.com/t-mart/kill-sticky
Or, a Firefox extension that adds a toolbar button: https://addons.mozilla.org/en-US/firefox/addon/kill-sticky/