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diff --git a/browser/components/sessionstore/RunState.sys.mjs b/browser/components/sessionstore/RunState.sys.mjs
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+++ b/browser/components/sessionstore/RunState.sys.mjs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const STATE_STOPPED = 0;
+const STATE_RUNNING = 1;
+const STATE_QUITTING = 2;
+const STATE_CLOSING = 3;
+const STATE_CLOSED = 4;
+
+// We're initially stopped.
+var state = STATE_STOPPED;
+
+/**
+ * This module keeps track of SessionStore's current run state. We will
+ * always start out at STATE_STOPPED. After the session was read from disk and
+ * the initial browser window has loaded we switch to STATE_RUNNING. On the
+ * first notice that a browser shutdown was granted we switch to STATE_QUITTING.
+ */
+export var RunState = Object.freeze({
+  // If we're stopped then SessionStore hasn't been initialized yet. As soon
+  // as the session is read from disk and the initial browser window has loaded
+  // the run state will change to STATE_RUNNING.
+  get isStopped() {
+    return state == STATE_STOPPED;
+  },
+
+  // STATE_RUNNING is our default mode of operation that we'll spend most of
+  // the time in. After the session was read from disk and the first browser
+  // window has loaded we remain running until the browser quits.
+  get isRunning() {
+    return state == STATE_RUNNING;
+  },
+
+  // We will enter STATE_QUITTING as soon as we receive notice that a browser
+  // shutdown was granted. SessionStore will use this information to prevent
+  // us from collecting partial information while the browser is shutting down
+  // as well as to allow a last single write to disk and block all writes after
+  // that.
+  get isQuitting() {
+    return state >= STATE_QUITTING;
+  },
+
+  // We will enter STATE_CLOSING as soon as SessionStore is uninitialized.
+  // The SessionFile module will know that a last write will happen in this
+  // state and it can do some necessary cleanup.
+  get isClosing() {
+    return state == STATE_CLOSING;
+  },
+
+  // We will enter STATE_CLOSED as soon as SessionFile has written to disk for
+  // the last time before shutdown and will not accept any further writes.
+  get isClosed() {
+    return state == STATE_CLOSED;
+  },
+
+  // Switch the run state to STATE_RUNNING. This must be called after the
+  // session was read from, the initial browser window has loaded and we're
+  // now ready to restore session data.
+  setRunning() {
+    if (this.isStopped) {
+      state = STATE_RUNNING;
+    }
+  },
+
+  // Switch the run state to STATE_CLOSING. This must be called *before* the
+  // last SessionFile.write() call so that SessionFile knows we're closing and
+  // can do some last cleanups and write a proper sessionstore.js file.
+  setClosing() {
+    if (this.isQuitting) {
+      state = STATE_CLOSING;
+    }
+  },
+
+  // Switch the run state to STATE_CLOSED. This must be called by SessionFile
+  // after the last write to disk was accepted and no further writes will be
+  // allowed. Any writes after this stage will cause exceptions.
+  setClosed() {
+    if (this.isClosing) {
+      state = STATE_CLOSED;
+    }
+  },
+
+  // Switch the run state to STATE_QUITTING. This should be called once we're
+  // certain that the browser is going away and before we start collecting the
+  // final window states to save in the session file.
+  setQuitting() {
+    if (this.isRunning) {
+      state = STATE_QUITTING;
+    }
+  },
+});