diff --git a/dom/ipc/TabParent.cpp b/dom/ipc/TabParent.cpp
index f876ec222093..0dba77992d74 100644
--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -118,6 +118,7 @@ public:
 private:
     ~OpenFileAndSendFDRunnable()
     {
+        MOZ_ASSERT(NS_IsMainThread());
         MOZ_ASSERT(!mFD);
     }
 
@@ -170,7 +171,8 @@ private:
         }
     }
 
-    void OpenFile()
+    // Helper method to avoid gnarly control flow for failures.
+    void OpenFileImpl()
     {
         MOZ_ASSERT(!NS_IsMainThread());
         MOZ_ASSERT(!mFD);
@@ -184,10 +186,21 @@ private:
         NS_ENSURE_SUCCESS_VOID(rv);
 
         mFD = fd;
+    }
+
+    void OpenFile()
+    {
+        MOZ_ASSERT(!NS_IsMainThread());
+
+        OpenFileImpl();
 
         if (NS_FAILED(NS_DispatchToMainThread(this))) {
             NS_WARNING("Failed to dispatch to main thread!");
 
+            // Intentionally leak the runnable (but not the fd) rather
+            // than crash when trying to release a main thread object
+            // off the main thread.
+            NS_ADDREF(this);
             CloseFile();
         }
     }