--- a/caps/nsPrincipal.cpp
+++ b/caps/nsPrincipal.cpp
@@ -341,16 +341,27 @@ nsPrincipal::CheckMayLoad(nsIURI* aURI, 
    if (aAllowIfInheritsPrincipal) {
     // If the caller specified to allow loads of URIs that inherit
     // our principal, allow the load if this URI inherits its principal
     if (nsPrincipal::IsPrincipalInherited(aURI)) {
       return NS_OK;
     }
   }
 
+  // See if aURI is something like a Blob URI that is actually associated with
+  // a principal.
+  nsCOMPtr<nsIURIWithPrincipal> uriWithPrin = do_QueryInterface(aURI);
+  nsCOMPtr<nsIPrincipal> uriPrin;
+  if (uriWithPrin) {
+    uriWithPrin->GetPrincipal(getter_AddRefs(uriPrin));
+  }
+  if (uriPrin && nsIPrincipal::Subsumes(uriPrin)) {
+      return NS_OK;
+  }
+
   if (nsScriptSecurityManager::SecurityCompareURIs(mCodebase, aURI)) {
     return NS_OK;
   }
 
   // If strict file origin policy is in effect, local files will always fail
   // SecurityCompareURIs unless they are identical. Explicitly check file origin
   // policy, in that case.
   if (nsScriptSecurityManager::GetStrictFileOriginPolicy() &&