--- a/browser/components/tests/startupRecorder.js
+++ b/browser/components/tests/startupRecorder.js
@@ -41,18 +41,17 @@ startupRecorder.prototype = {
   QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver]),
 
   record(name) {
     this.data.code[name] = {
       components: this.loader.loadedComponents(),
       modules: this.loader.loadedModules(),
       services: Object.keys(Cc).filter(c => {
         try {
-          Cm.isServiceInstantiatedByContractID(c, Ci.nsISupports);
-          return true;
+          return Cm.isServiceInstantiatedByContractID(c, Ci.nsISupports);
         } catch (e) {
           return false;
         }
       })
     };
   },
 
   observe(subject, topic, data) {

--- a/xpcom/components/nsComponentManager.cpp
+++ b/xpcom/components/nsComponentManager.cpp
@@ -1306,28 +1306,30 @@ nsComponentManagerImpl::IsServiceInstant
     aClass.ToProvidedString(cid);
     aIID.ToProvidedString(iid);
     fprintf(stderr, "Checking for service on shutdown. Denied.\n"
             "         CID: %s\n         IID: %s\n", cid, iid);
 #endif /* SHOW_DENIED_ON_SHUTDOWN */
     return NS_ERROR_UNEXPECTED;
   }
 
-  nsresult rv = NS_ERROR_SERVICE_NOT_AVAILABLE;
+  nsresult rv = NS_OK;
   nsFactoryEntry* entry;
 
   {
     SafeMutexAutoLock lock(mLock);
     entry = mFactories.Get(aClass);
   }
 
   if (entry && entry->mServiceObject) {
     nsCOMPtr<nsISupports> service;
     rv = entry->mServiceObject->QueryInterface(aIID, getter_AddRefs(service));
     *aResult = (service != nullptr);
+  } else {
+    *aResult = false;
   }
 
   return rv;
 }
 
 NS_IMETHODIMP
 nsComponentManagerImpl::IsServiceInstantiatedByContractID(
     const char* aContractID,
@@ -1346,27 +1348,29 @@ nsComponentManagerImpl::IsServiceInstant
     char iid[NSID_LENGTH];
     aIID.ToProvidedString(iid);
     fprintf(stderr, "Checking for service on shutdown. Denied.\n"
             "  ContractID: %s\n         IID: %s\n", aContractID, iid);
 #endif /* SHOW_DENIED_ON_SHUTDOWN */
     return NS_ERROR_UNEXPECTED;
   }
 
-  nsresult rv = NS_ERROR_SERVICE_NOT_AVAILABLE;
+  nsresult rv = NS_OK;
   nsFactoryEntry* entry;
   {
     SafeMutexAutoLock lock(mLock);
     entry = mContractIDs.Get(nsDependentCString(aContractID));
   }
 
   if (entry && entry->mServiceObject) {
     nsCOMPtr<nsISupports> service;
     rv = entry->mServiceObject->QueryInterface(aIID, getter_AddRefs(service));
     *aResult = (service != nullptr);
+  } else {
+    *aResult = false;
   }
   return rv;
 }
 
 
 NS_IMETHODIMP
 nsComponentManagerImpl::GetServiceByContractID(const char* aContractID,
                                                const nsIID& aIID,

--- a/xpcom/components/nsIServiceManager.idl
+++ b/xpcom/components/nsIServiceManager.idl
@@ -40,23 +40,22 @@ interface nsIServiceManager : nsISupport
     void getServiceByContractID(in string aContractID,
 				in nsIIDRef aIID, 
 				[iid_is(aIID),retval] out nsQIResult result);
 
     /**
      * isServiceInstantiated
      *
      * isServiceInstantiated will return a true if the service has already
-     * been created, or throw otherwise
+     * been created, or false otherwise. Throws if the service does not
+     * implement the given IID.
      *
      * @param aClass or aContractID : aClass or aContractID of object 
      *                                instance requested
      * @param aIID : IID of interface requested
-     * @throws NS_ERROR_SERVICE_NOT_AVAILABLE if the service hasn't been 
-     *         instantiated
      * @throws NS_NOINTERFACE if the IID given isn't supported by the object
      */
     boolean isServiceInstantiated(in nsCIDRef aClass, in nsIIDRef aIID);
     boolean isServiceInstantiatedByContractID(in string aContractID, in nsIIDRef aIID);
 };
 
 
 %{C++