--- a/dom/plugins/base/nsIPluginTag.idl
+++ b/dom/plugins/base/nsIPluginTag.idl
@@ -2,17 +2,17 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsISupports.idl"
 
 interface nsIURI;
 
-[scriptable, uuid(5daa99d5-265a-4397-b429-c943803e2619)]
+[builtinclass, scriptable, uuid(5daa99d5-265a-4397-b429-c943803e2619)]
 interface nsIPluginTag : nsISupports
 {
   // enabledState is stored as one of the following as an integer in prefs,
   // so if new states are added, they must not renumber the existing states.
   const unsigned long STATE_DISABLED = 0;
   const unsigned long STATE_CLICKTOPLAY = 1;
   const unsigned long STATE_ENABLED = 2;
 
@@ -25,31 +25,38 @@ interface nsIPluginTag : nsISupports
   // The 'nice' name of this plugin, e.g. 'flash' 'java'
   readonly attribute AUTF8String niceName;
 
   /**
    * true only if this plugin is "hardblocked" and cannot be enabled.
    */
   // FIXME-jsplugins QI to fakePluginTag possible
   // FIXME-jsplugins implement missing + tests (whatever that means)
+  [infallible]
   readonly attribute boolean blocklisted;
 
   /**
    * true if the state is non-default and locked, false otherwise.
    */
+  [infallible]
   readonly attribute boolean isEnabledStateLocked;
 
   // If this plugin is capable of being used (not disabled, blocklisted, etc)
+  [infallible]
   readonly attribute boolean active;
 
   // Get a specific nsIBlocklistService::STATE_*
+  [infallible]
   readonly attribute unsigned long blocklistState;
 
+  [infallible]
   readonly attribute boolean disabled;
+  [infallible]
   readonly attribute boolean clicktoplay;
+  [infallible]
   readonly attribute boolean loaded;
   // See the STATE_* values above.
            attribute unsigned long enabledState;
 
   readonly attribute PRTime lastModifiedTime;
 
   void getMimeTypes([optional] out unsigned long aCount,
                     [retval, array, size_is(aCount)] out wstring aResults);
@@ -61,17 +68,17 @@ interface nsIPluginTag : nsISupports
                      out wstring aResults);
 };
 
 /**
  * An interface representing a "fake" plugin: one implemented in JavaScript, not
  * as a NPAPI plug-in.  See nsIPluginHost.registerFakePlugin and the
  * documentation for the FakePluginTagInit dictionary.
  */
-[scriptable, uuid(6d22c968-226d-4156-b230-da6ad6bbf6e8)]
+[builtinclass, scriptable, uuid(6d22c968-226d-4156-b230-da6ad6bbf6e8)]
 interface nsIFakePluginTag : nsIPluginTag
 {
   /**
    * The URI that should be loaded into the tag (as a frame) to handle the
    * plugin. Note that the original data/src value for the plugin is not loaded
    * and will need to be requested by the handler via XHR or similar if desired.
    */
   readonly attribute nsIURI handlerURI;

--- a/dom/plugins/base/nsPluginHost.cpp
+++ b/dom/plugins/base/nsPluginHost.cpp
@@ -300,18 +300,17 @@ class BlocklistPromiseHandler final : pu
         MOZ_ASSERT(false, "Blocklist should always return int32");
         return;
       }
       int32_t newState = aValue.toInt32();
       MOZ_ASSERT(newState >= 0 && newState < nsIBlocklistService::STATE_MAX,
         "Shouldn't get an out of bounds blocklist state");
 
       // Check the old and new state and see if there was a change:
-      uint32_t oldState = nsIBlocklistService::STATE_NOT_BLOCKED;
-      MOZ_ALWAYS_SUCCEEDS(mTag->GetBlocklistState(&oldState));
+      uint32_t oldState = mTag->GetBlocklistState();
       bool changed = oldState != (uint32_t)newState;
       mTag->SetBlocklistState(newState);
 
       if (newState == nsIBlocklistService::STATE_SOFTBLOCKED && mShouldDisableWhenSoftblocked) {
         mTag->SetEnabledState(nsIPluginTag::STATE_DISABLED);
         changed = true;
       }
       sPluginBlocklistStatesChangedSinceLastWrite |= changed;