diff -r 000000000000 -r 6474c204b198 browser/components/sessionstore/test/unit/test_backup_once.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/sessionstore/test/unit/test_backup_once.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,48 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+let toplevel = this;
+Cu.import("resource://gre/modules/osfile.jsm");
+
+function run_test() {
+  let profd = do_get_profile();
+  Cu.import("resource:///modules/sessionstore/SessionFile.jsm", toplevel);
+  decoder = new TextDecoder();
+  pathStore = OS.Path.join(OS.Constants.Path.profileDir, "sessionstore.js");
+  pathBackup = OS.Path.join(OS.Constants.Path.profileDir, "sessionstore.bak");
+  let source = do_get_file("data/sessionstore_valid.js");
+  source.copyTo(profd, "sessionstore.js");
+  run_next_test();
+}
+
+let pathStore;
+let pathBackup;
+let decoder;
+
+// Write to the store, and check that a backup is created first
+add_task(function test_first_write_backup() {
+  let content = "test_1";
+  let initial_content = decoder.decode(yield OS.File.read(pathStore));
+
+  do_check_true(!(yield OS.File.exists(pathBackup)));
+  yield SessionFile.write(content);
+  do_check_true(yield OS.File.exists(pathBackup));
+
+  let backup_content = decoder.decode(yield OS.File.read(pathBackup));
+  do_check_eq(initial_content, backup_content);
+});
+
+// Write to the store again, and check that the backup is not updated
+add_task(function test_second_write_no_backup() {
+  let content = "test_2";
+  let initial_content = decoder.decode(yield OS.File.read(pathStore));
+  let initial_backup_content = decoder.decode(yield OS.File.read(pathBackup));
+
+  yield SessionFile.write(content);
+
+  let written_content = decoder.decode(yield OS.File.read(pathStore));
+  do_check_eq(content, written_content);
+
+  let backup_content = decoder.decode(yield OS.File.read(pathBackup));
+  do_check_eq(initial_backup_content, backup_content);
+});