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Seems like the Firefox add-ons intermediate signing cert just expired: bugzilla.mozilla.org/show_bug.cgi?id=1548973 Quick fix to allow installed add-ons to load again is setting xpinstall.signatures.required to false. However, that is obviously unsafe, so don't download any new ones in this state.
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While this setting is false, I recommend to disable auto-updating add-ons as well.
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After updating to 66.0.4, don't forget to enable signature checking again.