diff --git a/dom/ipc/ContentParent.cpp b/dom/ipc/ContentParent.cpp
index 511076f79bee..e1efe7cef759 100644
--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -631,8 +631,6 @@ ContentParent::PreallocateProcess()
                       eNotRecordingOrReplaying,
                       /* aRecordingFile = */ EmptyString());
 
-  PreallocatedProcessManager::AddBlocker(process);
-
   if (!process->LaunchSubprocess(PROCESS_PRIORITY_PREALLOC)) {
     return nullptr;
   }
@@ -902,13 +900,13 @@ ContentParent::GetNewOrUsedBrowserProcess(Element* aFrameElement,
   // Create a new process from scratch.
   RefPtr<ContentParent> p = new ContentParent(aOpener, aRemoteType, recordReplayState, recordingFile);
 
-  // Until the new process is ready let's not allow to start up any preallocated processes.
-  PreallocatedProcessManager::AddBlocker(p);
-
   if (!p->LaunchSubprocess(aPriority)) {
     return nullptr;
   }
 
+  // Until the new process is ready let's not allow to start up any preallocated processes.
+  PreallocatedProcessManager::AddBlocker(p);
+
   if (recordReplayState == eNotRecordingOrReplaying) {
     contentParents.AppendElement(p);
   }
diff --git a/dom/ipc/PreallocatedProcessManager.cpp b/dom/ipc/PreallocatedProcessManager.cpp
index e70d6cf60635..e6a3567ba6ee 100644
--- a/dom/ipc/PreallocatedProcessManager.cpp
+++ b/dom/ipc/PreallocatedProcessManager.cpp
@@ -212,7 +212,13 @@ void
 PreallocatedProcessManagerImpl::RemoveBlocker(ContentParent* aParent)
 {
   uint64_t childID = aParent->ChildID();
-  MOZ_ASSERT(mBlockers.Contains(childID));
+  // This used to assert that the blocker existed, but preallocated
+  // processes aren't blockers anymore because it's not useful and
+  // interferes with async launch, and it's simpler if content
+  // processes don't need to remember whether they were preallocated.
+  // (And preallocated processes can't AddBlocker when taken, because
+  // it's possible for a short-lived process to be recycled through
+  // Provide() and Take() before reaching RecvFirstIdle.)
   mBlockers.RemoveEntry(childID);
   if (!mPreallocatedProcess && mBlockers.IsEmpty()) {
     AllocateAfterDelay();