diff --git a/lib/firefox/webdriver/firefoxDelegate.js b/lib/firefox/webdriver/firefoxDelegate.js
index <HASH>..<HASH> 100644
--- a/lib/firefox/webdriver/firefoxDelegate.js
+++ b/lib/firefox/webdriver/firefoxDelegate.js
@@ -216,8 +216,14 @@ class FirefoxDelegate {
   async afterCollect(index, result) {
     for (let i = 0; i < result.length; i++) {
       if (!this.options.firefox.appconstants) {
-        delete result[i].browserScripts.browser.appConstants;
-        delete result[i].browserScripts.browser.asyncAppConstants;
+        // We need to be extra careful here, see
+        // https://github.com/sitespeedio/browsertime/issues/1261
+        if (result[i].browserScripts.browser.appConstants) {
+          delete result[i].browserScripts.browser.appConstants;
+        }
+        if (result[i].browserScripts.browser.asyncAppConstants) {
+          delete result[i].browserScripts.browser.asyncAppConstants;
+        }
       }
 
       if (this.firefoxConfig.geckoProfiler && this.options.visualMetrics) {
Verify that the FF data exist before deleting it #<I> (#<I>)