Bug 1587711

diff --git a/toolkit/mozapps/extensions/AddonManager.jsm b/toolkit/mozapps/extensions/AddonManager.jsm
--- a/toolkit/mozapps/extensions/AddonManager.jsm
+++ b/toolkit/mozapps/extensions/AddonManager.jsm
@@ -2328,17 +2328,18 @@ var AddonManagerInternal = {
           topBrowser,
           aInstallingPrincipal.URI,
           aInstall
         );
         return;
       } else if (
         aInstallingPrincipal.isNullPrincipal ||
         !aBrowser.contentPrincipal ||
-        !aInstallingPrincipal.subsumes(aBrowser.contentPrincipal) ||
+        (!aBrowser.contentPrincipal.isNullPrincipal &&
+          !aInstallingPrincipal.subsumes(aBrowser.contentPrincipal)) ||
         !this.isInstallAllowedByPolicy(
           aInstallingPrincipal,
           aInstall,
           false /* explicit */
         )
       ) {
         aInstall.cancel();