There's a new "experimental feature" being tested in Firefox 83: sponsored sites in the URL bar.
"Mozilla works with advertising partners to place sponsored tiles on the Firefox home page (or New Tab) that would be useful to Firefox users. Mozilla is paid when users click on sponsored tiles."
You can disable this by setting `browser.newtabpage.activity-stream.showSponsoredTopSites` to `false`.
Found via https://old.reddit.com/r/firefox/comments/jybx2w/uh_why_is_firefox_showing_me_sponsored_links_in/
@andreas Am I actually going to have to use EWW as my main browser?
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