--- a/toolkit/components/extensions/ExtensionContent.jsm
+++ b/toolkit/components/extensions/ExtensionContent.jsm
@@ -319,17 +319,17 @@ class Script {
 
     let scriptsPromise = Promise.all(this.compileScripts());
 
     // If we're supposed to inject at the start of the document load,
     // and we haven't already missed that point, block further parsing
     // until the scripts have been loaded.
     let {document} = context.contentWindow;
     if (this.runAt === "document_start" && document.readyState !== "complete") {
-      document.blockParsing(scriptsPromise);
+      document.blockParsing(scriptsPromise, {blockScriptCreated: false});
     }
 
     let scripts = await scriptsPromise;
     let result;
 
     // The evaluations below may throw, in which case the promise will be
     // automatically rejected.
     TelemetryStopwatch.start(CONTENT_SCRIPT_INJECTION_HISTOGRAM, context);