Some websites (like Yahoo) load *so much crap* that they defeat all of these countermeasures. For these websites, I use the "Element Blocker" plug-in, which lets you delete parts of the web-page, either for a single session, or permanently:
addons.mozilla.org/en-US/firefo...
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Element Blocker – Get this Extension for 🦊 Firefox (en-US)
Download Element Blocker for Firefox. Block popups, banners, chat widgets, notification bars, and more from appearing on any website.