--- a/browser/components/sessionstore/content/content-sessionStore.js
+++ b/browser/components/sessionstore/content/content-sessionStore.js
@@ -708,16 +708,18 @@ var MessageQueue = {
         for (let histogramId of Object.keys(value)) {
           telemetry[histogramId] = value[histogramId];
         }
       } else {
         data[key] = value;
       }
     }
 
+    this._data.clear();
+
     durationMs = Date.now() - durationMs;
     telemetry.FX_SESSION_RESTORE_CONTENT_COLLECT_DATA_LONGEST_OP_MS = durationMs;
 
     try {
       // Send all data to the parent process.
       sendAsyncMessage("SessionStore:update", {
         data, telemetry, flushID,
         isFinal: options.isFinal || false,