reviewed/signed add-on enforcement in Firefox 42
According to this Mozilla Add-ons Blog entry,
Starting in Firefox 42, add-on developers will be required to submit extensions for review and signing by Mozilla prior to deployment, and unsigned add-ons cannot be installed or used with Firefox.This Mozilla Wiki page has further relevant details — e.g. the current plan (presumably not yet set in stone) is that the ESR branch starting in Firefox 45 ESR will act like Firefox 41 (only) in that the enforcement mechanism can be overridden by a preference in
If you use Firefox and depend upon unsigned add-ons (e.g. those not available through addons.mozilla.org), start thinking about what your mitigation strategy will be.