--- a/browser/components/sessionstore/src/SessionStore.jsm
+++ b/browser/components/sessionstore/src/SessionStore.jsm
@@ -3608,20 +3608,22 @@ let SessionStoreInternal = {
    *        Bool update all windows
    */
   saveState: function ssi_saveState(aUpdateAll) {
     // If crash recovery is disabled, we only want to resume with pinned tabs
     // if we crash.
     let pinnedOnly = this._loadState == STATE_RUNNING && !this._resume_from_crash;
 
     TelemetryStopwatch.start("FX_SESSION_RESTORE_COLLECT_DATA_MS");
+    TelemetryStopwatch.start("FX_SESSION_RESTORE_COLLECT_DATA_LONGEST_OP_MS");
 
     var oState = this._getCurrentState(aUpdateAll, pinnedOnly);
     if (!oState) {
       TelemetryStopwatch.cancel("FX_SESSION_RESTORE_COLLECT_DATA_MS");
+      TelemetryStopwatch.cancel("FX_SESSION_RESTORE_COLLECT_DATA_LONGEST_OP_MS");
       return;
     }
 
     // Forget about private windows.
     for (let i = oState.windows.length - 1; i >= 0; i--) {
       if (oState.windows[i].isPrivate) {
         oState.windows.splice(i, 1);
         if (oState.selectedWindow >= i) {
@@ -3665,27 +3667,30 @@ let SessionStoreInternal = {
       }
     }
 
     // Persist the last session if we deferred restoring it
     if (this._lastSessionState)
       oState.lastSessionState = this._lastSessionState;
 
     TelemetryStopwatch.finish("FX_SESSION_RESTORE_COLLECT_DATA_MS");
+    TelemetryStopwatch.finish("FX_SESSION_RESTORE_COLLECT_DATA_LONGEST_OP_MS");
 
     this._saveStateObject(oState);
   },
 
   /**
    * write a state object to disk
    */
   _saveStateObject: function ssi_saveStateObject(aStateObj) {
     TelemetryStopwatch.start("FX_SESSION_RESTORE_SERIALIZE_DATA_MS");
+    TelemetryStopwatch.start("FX_SESSION_RESTORE_SERIALIZE_DATA_LONGEST_OP_MS");
     let data = this._toJSONString(aStateObj);
     TelemetryStopwatch.finish("FX_SESSION_RESTORE_SERIALIZE_DATA_MS");
+    TelemetryStopwatch.finish("FX_SESSION_RESTORE_SERIALIZE_DATA_LONGEST_OP_MS");
 
     let stateString = this._createSupportsString(data);
     Services.obs.notifyObservers(stateString, "sessionstore-state-write", "");
     data = stateString.data;
 
     // Don't touch the file if an observer has deleted all state data.
     if (!data) {
       return;

--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -2137,22 +2137,34 @@
     "description": "Widget: Time it takes for the message before a UI message (ms)"
   },
   "FX_SESSION_RESTORE_COLLECT_DATA_MS": {
     "kind": "exponential",
     "high": "30000",
     "n_buckets": 10,
     "description": "Session restore: Time to collect all window and tab data (ms)"
   },
+  "FX_SESSION_RESTORE_COLLECT_DATA_LONGEST_OP_MS": {
+    "kind": "exponential",
+    "high": "30000",
+    "n_buckets": 10,
+    "description": "Session restore: Duration of the longest uninterruptible operation while collecting all window and tab data (ms)"
+  },
   "FX_SESSION_RESTORE_SERIALIZE_DATA_MS": {
     "kind": "exponential",
     "high": "1000",
     "n_buckets": 10,
     "description": "Session restore: Time to JSON serialize session data (ms)"
   },
+  "FX_SESSION_RESTORE_SERIALIZE_DATA_LONGEST_OP_MS": {
+    "kind": "exponential",
+    "high": "30000",
+    "n_buckets": 10,
+    "description": "Session restore: Duration of the longest uninterruptible operation while serializing session data (ms)"
+  },
   "FX_SESSION_RESTORE_READ_FILE_MS": {
     "kind": "exponential",
     "high": "3000",
     "n_buckets": 10,
     "description": "Session restore: Time to read the session data from the file on disk (ms)"
   },
   "FX_SESSION_RESTORE_SYNC_READ_FILE_MS": {
     "kind": "exponential",