Firefox only shims them when they would otherwise be blocked by strict or private browsing's tracking protection. It does so because when they are simply blocked, quite a few sites will break (sometimes in minor ways, and sometimes completely). Whether that's a good or bad thing is entirely up to you. You are free to disable SmartBlock if you'd prefer not to activate any shims (in `about:config`, the `extensions.webcompat.enable_shims` setting) at the expense of more websites breaking while using stricter tracking protection.
Bug 1713687 Comment 8 Edit History
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Firefox only shims them when they would otherwise be blocked by strict or private browsing's tracking protection. It does so because when they are simply blocked, quite a few sites will break (sometimes in minor ways, and sometimes completely). Whether that's a good or bad thing is entirely up to you. You are free to disable SmartBlock if you'd prefer not to activate any shims (in `about:config`, the `extensions.webcompat.enable_shims` setting) at the expense of more websites breaking while using stricter tracking protection. Hope that helps!