From 1e3b008bd7647860b9031a885e1c2b598dc27437 Mon Sep 17 00:00:00 2001
From: Dave Townsend <dtownsend@oxymoronical.com>
Date: Wed, 30 Jan 2019 14:56:30 -0800
Subject: [PATCH] Bug 1455707: Detect when running an older version than
 previously ran with the selected profile. r=froydnj, r=mconley, r=Gijs

Use the information in compatibility.ini to detect that the current running
application is an older version than previously ran with the profile and in
that case open a UI allowing the user to launch the profile manager, launch
the previous instance of the application or quit.

Also includes the patch from bug 1523725.

--HG--
rename : browser/themes/shared/information.svg => toolkit/themes/shared/profile/information.svg
extra : rebase_source : 3bf8b329eb5ea9e71fe2f0ed34a7e44dfdc434fd
extra : intermediate-source : 21a801ca5f6d435509f93e1dee187cb6ca868c8f
extra : source : c9d89812bc226ca593119bf440cb4f5e50ac2ace
---
 browser/themes/shared/jar.inc.mn              |   1 -
 browser/themes/shared/newInstall.css          |   2 +-
 .../mozapps/profile/profileDowngrade.dtd      |  20 ++
 toolkit/locales/jar.mn                        |   3 +
 toolkit/profile/content/profileDowngrade.js   |  29 ++
 toolkit/profile/content/profileDowngrade.xul  |  39 +++
 toolkit/profile/jar.mn                        |   4 +
 toolkit/profile/nsIToolkitProfileService.idl  |  39 ++-
 toolkit/profile/nsToolkitProfileService.cpp   |  24 +-
 toolkit/themes/shared/mozapps.inc.mn          |   2 +
 .../themes/shared/profile}/information.svg    |   0
 .../shared/profile/profileDowngrade.css       |  13 +
 toolkit/xre/nsAppRunner.cpp                   | 254 +++++++++++++++---
 13 files changed, 396 insertions(+), 34 deletions(-)
 create mode 100644 toolkit/locales/en-US/chrome/mozapps/profile/profileDowngrade.dtd
 create mode 100644 toolkit/profile/content/profileDowngrade.js
 create mode 100644 toolkit/profile/content/profileDowngrade.xul
 rename {browser/themes/shared => toolkit/themes/shared/profile}/information.svg (100%)
 create mode 100644 toolkit/themes/shared/profile/profileDowngrade.css

diff --git a/browser/themes/shared/jar.inc.mn b/browser/themes/shared/jar.inc.mn
index 670d8f39a192..181d284bf5b9 100644
--- a/browser/themes/shared/jar.inc.mn
+++ b/browser/themes/shared/jar.inc.mn
@@ -56,7 +56,6 @@
   skin/classic/browser/identity-icon.svg                       (../shared/identity-block/identity-icon.svg)
   skin/classic/browser/identity-icon-notice.svg                (../shared/identity-block/identity-icon-notice.svg)
   skin/classic/browser/info.svg                                (../shared/info.svg)
-  skin/classic/browser/information.svg                         (../shared/information.svg)
   skin/classic/browser/newInstall.css                          (../shared/newInstall.css)
   skin/classic/browser/newInstallPage.css                      (../shared/newInstallPage.css)
 
diff --git a/browser/themes/shared/newInstall.css b/browser/themes/shared/newInstall.css
index a9d9dbc0d05f..33daa754d96f 100644
--- a/browser/themes/shared/newInstall.css
+++ b/browser/themes/shared/newInstall.css
@@ -10,7 +10,7 @@ window {
   width: 32px;
   height: 32px;
   margin-inline-end: 8px;
-  list-style-image: url("chrome://browser/skin/information.svg");
+  list-style-image: url("chrome://mozapps/skin/profile/information.svg");
 }
 
 description {
diff --git a/toolkit/locales/en-US/chrome/mozapps/profile/profileDowngrade.dtd b/toolkit/locales/en-US/chrome/mozapps/profile/profileDowngrade.dtd
new file mode 100644
index 000000000000..ce3b8388bd87
--- /dev/null
+++ b/toolkit/locales/en-US/chrome/mozapps/profile/profileDowngrade.dtd
@@ -0,0 +1,20 @@
+<!-- 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/. -->
+
+<!-- LOCALIZATION NOTE:
+ This UI can be most easily shown by modifying the version in compatibility.ini
+ to a newer version and then starting Firefox.
+ For this feature, "installation" is used to mean "this discrete download of
+ Firefox" and "version" is used to mean "the specific revision number of a
+ given Firefox channel". These terms are not synonymous.
+-->
+<!ENTITY window.title "You’ve launched an older version of Firefox">
+<!ENTITY window.style "width: 490px;">
+
+<!ENTITY window.nosync "Using an older version of Firefox can corrupt bookmarks and browsing history already saved to an existing Firefox profile. To protect your information, create a new profile for this installation of &brandShortName;.">
+<!ENTITY window.sync "Using an older version of Firefox can corrupt bookmarks and browsing history already saved to an existing Firefox profile. To protect your information, create a new profile for this installation of &brandShortName;. You can always sign in with a &syncBrand.fxAccount.label; to sync your bookmarks and browsing history between profiles.">
+
+<!ENTITY window.create "Create New Profile">
+<!ENTITY window.quit-win "Exit">
+<!ENTITY window.quit-nonwin "Quit">
diff --git a/toolkit/locales/jar.mn b/toolkit/locales/jar.mn
index a5c65f929eaf..daa821d3f02c 100644
--- a/toolkit/locales/jar.mn
+++ b/toolkit/locales/jar.mn
@@ -83,6 +83,9 @@
   locale/@AB_CD@/mozapps/profile/createProfileWizard.dtd          (%chrome/mozapps/profile/createProfileWizard.dtd)
   locale/@AB_CD@/mozapps/profile/profileSelection.properties      (%chrome/mozapps/profile/profileSelection.properties)
   locale/@AB_CD@/mozapps/profile/profileSelection.dtd             (%chrome/mozapps/profile/profileSelection.dtd)
+#ifdef MOZ_BLOCK_PROFILE_DOWNGRADE
+  locale/@AB_CD@/mozapps/profile/profileDowngrade.dtd             (%chrome/mozapps/profile/profileDowngrade.dtd)
+#endif
 #ifndef MOZ_FENNEC
   locale/@AB_CD@/mozapps/update/updates.dtd                       (%chrome/mozapps/update/updates.dtd)
   locale/@AB_CD@/mozapps/update/updates.properties                (%chrome/mozapps/update/updates.properties)
diff --git a/toolkit/profile/content/profileDowngrade.js b/toolkit/profile/content/profileDowngrade.js
new file mode 100644
index 000000000000..39f8195b0ab3
--- /dev/null
+++ b/toolkit/profile/content/profileDowngrade.js
@@ -0,0 +1,29 @@
+/* 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/. */
+
+let gParams;
+
+function init() {
+  /*
+   * The C++ code passes a dialog param block using its integers as in and out
+   * arguments for this UI. The following are the uses of the integers:
+   *
+   *  0: A set of flags from nsIToolkitProfileService.downgradeUIFlags.
+   *  1: A return argument, one of nsIToolkitProfileService.downgradeUIChoice.
+   */
+  gParams = window.arguments[0].QueryInterface(Ci.nsIDialogParamBlock);
+  let hasSync = gParams.GetInt(0) & Ci.nsIToolkitProfileService.hasSync;
+
+  document.getElementById("sync").hidden = !hasSync;
+  document.getElementById("nosync").hidden = hasSync;
+}
+
+function quit() {
+  gParams.SetInt(1, Ci.nsIToolkitProfileService.quit);
+}
+
+function createProfile() {
+  gParams.SetInt(1, Ci.nsIToolkitProfileService.createNewProfile);
+  window.close();
+}
diff --git a/toolkit/profile/content/profileDowngrade.xul b/toolkit/profile/content/profileDowngrade.xul
new file mode 100644
index 000000000000..4189fd6894c9
--- /dev/null
+++ b/toolkit/profile/content/profileDowngrade.xul
@@ -0,0 +1,39 @@
+<?xml version="1.0"?>
+<!-- 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/. -->
+
+<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+<?xml-stylesheet href="chrome://mozapps/skin/profile/profileDowngrade.css" type="text/css"?>
+
+<!DOCTYPE dialog [
+<!ENTITY % brandDTD SYSTEM "chrome://branding/locale/brand.dtd">
+%brandDTD;
+<!ENTITY % syncBrandDTD SYSTEM "chrome://browser/locale/syncBrand.dtd">
+%syncBrandDTD;
+<!ENTITY % profileDTD SYSTEM "chrome://mozapps/locale/profile/profileDowngrade.dtd">
+%profileDTD;
+]>
+
+<dialog xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        title="&window.title;" onload="init()" style="&window.style;"
+        buttonlabelextra1="&window.create;" ondialogextra1="createProfile()"
+#ifdef XP_WIN
+        buttonlabelaccept="&window.quit-win;"
+#else
+        buttonlabelaccept="&window.quit-nonwin;"
+#endif
+        ondialogaccept="quit()" ondialogcancel="quit()"
+        buttons="accept,extra1" buttonpack="end">
+
+  <script type="application/javascript" src="profileDowngrade.js"/>
+
+  <hbox flex="1" align="start">
+    <image id="info" role="presentation"/>
+    <vbox flex="1">
+      <description id="nosync">&window.nosync;</description>
+      <description id="sync">&window.sync;</description>
+    </vbox>
+  </hbox>
+
+</dialog>
diff --git a/toolkit/profile/jar.mn b/toolkit/profile/jar.mn
index 9b7c22266ebd..5620cb0000b5 100644
--- a/toolkit/profile/jar.mn
+++ b/toolkit/profile/jar.mn
@@ -7,3 +7,7 @@ toolkit.jar:
 * content/mozapps/profile/createProfileWizard.xul (content/createProfileWizard.xul)
   content/mozapps/profile/profileSelection.js     (content/profileSelection.js)
   content/mozapps/profile/profileSelection.xul    (content/profileSelection.xul)
+#ifdef MOZ_BLOCK_PROFILE_DOWNGRADE
+  content/mozapps/profile/profileDowngrade.js     (content/profileDowngrade.js)
+* content/mozapps/profile/profileDowngrade.xul    (content/profileDowngrade.xul)
+#endif
diff --git a/toolkit/profile/nsIToolkitProfileService.idl b/toolkit/profile/nsIToolkitProfileService.idl
index 00c8c228ab6a..8547ea7829d6 100644
--- a/toolkit/profile/nsIToolkitProfileService.idl
+++ b/toolkit/profile/nsIToolkitProfileService.idl
@@ -13,6 +13,23 @@ interface nsIProfileLock;
 [scriptable, builtinclass, uuid(1947899b-f369-48fa-89da-f7c37bb1e6bc)]
 interface nsIToolkitProfileService : nsISupports
 {
+    /**
+     * When a downgrade is detected UI is presented to the user to ask how to
+     * proceed. These flags are used to pass some information to the UI.
+     */
+    cenum downgradeUIFlags: 8 {
+        hasSync = 1,
+    };
+
+    /**
+     * When a downgrade is detected UI is presented to the user to ask how to
+     * proceed. These are the possible options the user can choose.
+     */
+    cenum downgradeUIChoice: 8 {
+        quit = 0,
+        createNewProfile = 1,
+    };
+
     attribute boolean startWithLastProfile;
 
     readonly attribute nsISimpleEnumerator /*nsIToolkitProfile*/ profiles;
@@ -74,7 +91,10 @@ interface nsIToolkitProfileService : nsISupports
      *
      * The profile temporary directory will be chosen based on where the
      * profile directory is located.
-     * 
+     *
+     * If a profile with the given name already exists it will be returned
+     * instead of creating a new profile.
+     *
      * @param aRootDir
      *        The profile directory. May be null, in which case a suitable
      *        default will be chosen based on the profile name.
@@ -84,6 +104,23 @@ interface nsIToolkitProfileService : nsISupports
     nsIToolkitProfile createProfile(in nsIFile aRootDir,
                                     in AUTF8String aName);
 
+    /**
+     * Create a new profile with a unique name.
+     *
+     * As above however the name given will be altered to make it a unique
+     * profile name.
+     *
+     * @param aRootDir
+     *        The profile directory. May be null, in which case a suitable
+     *        default will be chosen based on the profile name.
+     * @param aNamePrefix
+     *        The prefix to use for the profile name. If unused this will be
+     *        used as the profile name otherwise additional characters will be
+     *        added to make the name unique.
+     */
+    nsIToolkitProfile createUniqueProfile(in nsIFile aRootDir,
+                                          in AUTF8String aNamePrefix);
+
     /**
      * Returns the number of profiles.
      * @return the number of profiles.
diff --git a/toolkit/profile/nsToolkitProfileService.cpp b/toolkit/profile/nsToolkitProfileService.cpp
index 07c3aebbae00..79506052b1e9 100644
--- a/toolkit/profile/nsToolkitProfileService.cpp
+++ b/toolkit/profile/nsToolkitProfileService.cpp
@@ -1066,7 +1066,7 @@ nsresult nsToolkitProfileService::SelectStartupProfile(
       name.AssignLiteral(DEFAULT_NAME);
     }
 
-    rv = CreateProfile(nullptr, name, getter_AddRefs(mCurrent));
+    rv = CreateUniqueProfile(nullptr, name, getter_AddRefs(mCurrent));
     if (NS_SUCCEEDED(rv)) {
       if (mUseDedicatedProfile) {
         SetDefaultProfile(mCurrent);
@@ -1261,6 +1261,28 @@ static void SaltProfileName(nsACString& aName) {
   aName.Insert(salt, 0, 9);
 }
 
+NS_IMETHODIMP
+nsToolkitProfileService::CreateUniqueProfile(nsIFile* aRootDir,
+                                             const nsACString& aNamePrefix,
+                                             nsIToolkitProfile** aResult) {
+  nsCOMPtr<nsIToolkitProfile> profile;
+  nsresult rv = GetProfileByName(aNamePrefix, getter_AddRefs(profile));
+  if (NS_FAILED(rv)) {
+    return CreateProfile(aRootDir, aNamePrefix, aResult);
+  }
+
+  uint32_t suffix = 1;
+  while (true) {
+    nsPrintfCString name("%s-%d", PromiseFlatCString(aNamePrefix).get(),
+                         suffix);
+    rv = GetProfileByName(name, getter_AddRefs(profile));
+    if (NS_FAILED(rv)) {
+      return CreateProfile(aRootDir, name, aResult);
+    }
+    suffix++;
+  }
+}
+
 NS_IMETHODIMP
 nsToolkitProfileService::CreateProfile(nsIFile* aRootDir,
                                        const nsACString& aName,
diff --git a/toolkit/themes/shared/mozapps.inc.mn b/toolkit/themes/shared/mozapps.inc.mn
index e3813ac0925c..c1eb8daa0f47 100644
--- a/toolkit/themes/shared/mozapps.inc.mn
+++ b/toolkit/themes/shared/mozapps.inc.mn
@@ -28,6 +28,8 @@
   skin/classic/mozapps/aboutProfiles.css                     (../../shared/aboutProfiles.css)
 #endif
   skin/classic/mozapps/aboutServiceWorkers.css               (../../shared/aboutServiceWorkers.css)
+  skin/classic/mozapps/profile/profileDowngrade.css          (../../shared/profile/profileDowngrade.css)
+  skin/classic/mozapps/profile/information.svg               (../../shared/profile/information.svg)
 
 % override chrome://mozapps/skin/extensions/category-plugins.svg          chrome://global/skin/plugins/pluginGeneric.svg
 % override chrome://mozapps/skin/extensions/category-extensions.svg       chrome://mozapps/skin/extensions/extensionGeneric.svg
diff --git a/browser/themes/shared/information.svg b/toolkit/themes/shared/profile/information.svg
similarity index 100%
rename from browser/themes/shared/information.svg
rename to toolkit/themes/shared/profile/information.svg
diff --git a/toolkit/themes/shared/profile/profileDowngrade.css b/toolkit/themes/shared/profile/profileDowngrade.css
new file mode 100644
index 000000000000..d30fdb65d80d
--- /dev/null
+++ b/toolkit/themes/shared/profile/profileDowngrade.css
@@ -0,0 +1,13 @@
+/* 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/. */
+
+spacer[anonid="spacer"] {
+  display: none;
+}
+
+#info {
+  list-style-image: url("chrome://mozapps/skin/profile/information.svg");
+  width: 32px;
+  height: 32px;
+}
diff --git a/toolkit/xre/nsAppRunner.cpp b/toolkit/xre/nsAppRunner.cpp
index 74be7e80de4d..1ab0ac6dd9b9 100644
--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -1494,6 +1494,9 @@ static void DumpHelp() {
       "instance.\n"
       "  --UILocale <locale> Start with <locale> resources as UI Locale.\n"
       "  --safe-mode        Disables extensions and themes for this session.\n"
+#ifdef MOZ_BLOCK_PROFILE_DOWNGRADE
+      "  --allow-downgrade  Allows downgrading a profile.\n"
+#endif
       "  -MOZ_LOG=<modules> Treated as MOZ_LOG=<modules> environment variable, "
       "overrides it.\n"
       "  -MOZ_LOG_FILE=<file> Treated as MOZ_LOG_FILE=<file> environment "
@@ -2230,18 +2233,188 @@ static nsresult SelectProfile(nsIProfileLock** aResult,
   return ProfileLockedDialog(rootDir, localDir, unlocker, aNative, aResult);
 }
 
+#ifdef MOZ_BLOCK_PROFILE_DOWNGRADE
+static const char kProfileDowngradeURL[] =
+    "chrome://mozapps/content/profile/profileDowngrade.xul";
+
+static ReturnAbortOnError CheckDowngrade(
+    nsIFile* aProfileDir, nsIFile* aProfileLocalDir, nsACString& aProfileName,
+    nsINativeAppSupport* aNative, nsIToolkitProfileService* aProfileSvc) {
+  int32_t result = 0;
+  nsresult rv;
+
+  {
+    if (gfxPlatform::IsHeadless()) {
+      // TODO: make a way to turn off all dialogs when headless.
+      Output(true,
+             "This profile was last used with a newer version of this "
+             "application. Please create a new profile.\n");
+      return NS_ERROR_ABORT;
+    }
+
+    ScopedXPCOMStartup xpcom;
+    rv = xpcom.Initialize();
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    rv = xpcom.SetWindowCreator(aNative);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    {  // extra scoping is needed so we release these components before xpcom
+       // shutdown
+      bool hasSync = false;
+      nsCOMPtr<nsIPrefService> prefSvc =
+          do_GetService("@mozilla.org/preferences-service;1");
+      NS_ENSURE_TRUE(prefSvc, rv);
+
+      nsCOMPtr<nsIFile> prefsFile;
+      rv = aProfileDir->Clone(getter_AddRefs(prefsFile));
+      NS_ENSURE_SUCCESS(rv, rv);
+
+      rv = prefsFile->Append(NS_LITERAL_STRING("prefs.js"));
+      NS_ENSURE_SUCCESS(rv, rv);
+
+      rv = prefSvc->ReadUserPrefsFromFile(prefsFile);
+      if (NS_SUCCEEDED(rv)) {
+        nsCOMPtr<nsIPrefBranch> prefBranch = do_QueryInterface(prefSvc);
+
+        rv = prefBranch->PrefHasUserValue("services.sync.username", &hasSync);
+        NS_ENSURE_SUCCESS(rv, rv);
+      }
+
+      nsCOMPtr<nsIWindowWatcher> windowWatcher =
+          do_GetService(NS_WINDOWWATCHER_CONTRACTID);
+      NS_ENSURE_TRUE(windowWatcher, NS_ERROR_ABORT);
+
+      nsCOMPtr<nsIAppStartup> appStartup(components::AppStartup::Service());
+      NS_ENSURE_TRUE(appStartup, NS_ERROR_FAILURE);
+
+      nsCOMPtr<nsIDialogParamBlock> paramBlock =
+          do_CreateInstance(NS_DIALOGPARAMBLOCK_CONTRACTID);
+      NS_ENSURE_TRUE(paramBlock, NS_ERROR_ABORT);
+
+      uint8_t flags = 0;
+      if (hasSync) {
+        flags |= nsIToolkitProfileService::hasSync;
+      }
+
+      paramBlock->SetInt(0, flags);
+
+      nsCOMPtr<mozIDOMWindowProxy> newWindow;
+      rv = windowWatcher->OpenWindow(nullptr, kProfileDowngradeURL, "_blank",
+                                     "centerscreen,chrome,modal,titlebar",
+                                     paramBlock, getter_AddRefs(newWindow));
+      NS_ENSURE_SUCCESS(rv, rv);
+
+      paramBlock->GetInt(1, &result);
+    }
+  }
+
+  if (result == nsIToolkitProfileService::createNewProfile) {
+    // Create a new profile and start it.
+    nsCString profileName;
+    profileName.AssignLiteral("default");
+#  ifdef MOZ_DEDICATED_PROFILES
+    profileName.Append("-" NS_STRINGIFY(MOZ_UPDATE_CHANNEL));
+#  endif
+    nsCOMPtr<nsIToolkitProfile> newProfile;
+    rv = aProfileSvc->CreateUniqueProfile(nullptr, profileName,
+                                          getter_AddRefs(newProfile));
+    NS_ENSURE_SUCCESS(rv, rv);
+    rv = aProfileSvc->SetDefaultProfile(newProfile);
+    NS_ENSURE_SUCCESS(rv, rv);
+    rv = aProfileSvc->Flush();
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    nsCOMPtr<nsIFile> profD, profLD;
+    rv = newProfile->GetRootDir(getter_AddRefs(profD));
+    NS_ENSURE_SUCCESS(rv, rv);
+    rv = newProfile->GetLocalDir(getter_AddRefs(profLD));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    SaveFileToEnv("XRE_PROFILE_PATH", profD);
+    SaveFileToEnv("XRE_PROFILE_LOCAL_PATH", profLD);
+
+    return LaunchChild(aNative);
+  }
+
+  // Cancel
+  return NS_ERROR_ABORT;
+}
+#endif
+
+/**
+ * The version string used in compatibility.ini is in the form
+ * <appversion>_<appbuildid>/<aplatformbuildidid>. The build IDs are in the form
+ * <year><month><day><hour><minute><second>. We need to be able to turn this
+ * into something that can be compared by the version comparator. Build IDs are
+ * numeric so normally we could just use those as part of the version but they
+ * are larger than 32-bit integers so we must split them into two parts.
+ * So we generate a version string of the format:
+ * <appversion>.<appbuilddate>.<appbuildtime>.<platformbuilddate>.<platformbuildtime>
+ * This doesn't compare correctly so
+ * we have to make the build ids separate version parts instead. We also have
+ * to split up the build ids as they are too large for the version comparator's
+ * brain.
+ */
+
+// Build ID dates are 8 digits, build ID times are 6 digits.
+#define BUILDID_DATE_LENGTH 8
+#define BUILDID_TIME_LENGTH 6
+#define BUILDID_LENGTH BUILDID_DATE_LENGTH + BUILDID_TIME_LENGTH
+
+static void GetBuildIDVersions(const nsACString& aFullVersion, int32_t aPos,
+                               nsACString& aBuildVersions) {
+  // Extract the date part then the time part.
+  aBuildVersions.Assign(Substring(aFullVersion, aPos, BUILDID_DATE_LENGTH) +
+      NS_LITERAL_CSTRING(".") +
+      Substring(aFullVersion, aPos + BUILDID_DATE_LENGTH, BUILDID_TIME_LENGTH));
+}
+
+static Version GetComparableVersion(const nsCString& aVersionStr) {
+  // We'll find the position of the '_' and '/' characters. The length from the
+  // '_' character to the '/' character will be the length of a build ID plus
+  // one for the '_' character. Similarly the length from the '/' character to
+  // the end of the string will be the same.
+  const uint32_t kExpectedLength = BUILDID_LENGTH + 1;
+
+  int32_t underscorePos = aVersionStr.FindChar('_');
+  int32_t slashPos = aVersionStr.FindChar('/');
+  if (underscorePos == kNotFound || slashPos == kNotFound ||
+      (slashPos - underscorePos) != kExpectedLength ||
+      (aVersionStr.Length() - slashPos) != kExpectedLength) {
+    NS_WARNING("compatibility.ini Version string does not match the expected format.");
+    return Version(aVersionStr.get());
+  }
+
+  nsCString appBuild, platBuild;
+  NS_NAMED_LITERAL_CSTRING(dot, ".");
+
+  const nsACString& version = Substring(aVersionStr, 0, underscorePos);
+
+  GetBuildIDVersions(aVersionStr, underscorePos + 1, /* outparam */ appBuild);
+  GetBuildIDVersions(aVersionStr, slashPos + 1, /* outparam */ platBuild);
+
+  const nsACString& fullVersion = version + dot + appBuild + dot + platBuild;
+
+  return Version(PromiseFlatCString(fullVersion).get());
+}
+
 /**
  * Checks the compatibility.ini file to see if we have updated our application
  * or otherwise invalidated our caches. If the application has been updated,
  * we return false; otherwise, we return true. We also write the status
  * of the caches (valid/invalid) into the return param aCachesOK. The aCachesOK
- * is always invalid if the application has been updated.
+ * is always invalid if the application has been updated. aIsDowngrade is set to
+ * true if the current application is older than that previously used by the
+ * profile.
  */
 static bool CheckCompatibility(nsIFile* aProfileDir, const nsCString& aVersion,
                                const nsCString& aOSABI, nsIFile* aXULRunnerDir,
                                nsIFile* aAppDir, nsIFile* aFlagFile,
-                               bool* aCachesOK) {
+                               bool* aCachesOK, bool* aIsDowngrade) {
   *aCachesOK = false;
+  *aIsDowngrade = false;
+
   nsCOMPtr<nsIFile> file;
   aProfileDir->Clone(getter_AddRefs(file));
   if (!file) return false;
@@ -2253,7 +2426,16 @@ static bool CheckCompatibility(nsIFile* aProfileDir, const nsCString& aVersion,
 
   nsAutoCString buf;
   rv = parser.GetString("Compatibility", "LastVersion", buf);
-  if (NS_FAILED(rv) || !aVersion.Equals(buf)) return false;
+  if (NS_FAILED(rv)) {
+    return false;
+  }
+
+  if (!aVersion.Equals(buf)) {
+    Version current = GetComparableVersion(aVersion);
+    Version last = GetComparableVersion(buf);
+    *aIsDowngrade = last > current;
+    return false;
+  }
 
   rv = parser.GetString("Compatibility", "LastOSABI", buf);
   if (NS_FAILED(rv) || !aOSABI.Equals(buf)) return false;
@@ -3840,30 +4022,6 @@ int XREMain::XRE_mainStartup(bool* aExitFlag) {
   rv = mProfileLock->GetLocalDirectory(getter_AddRefs(mProfLD));
   NS_ENSURE_SUCCESS(rv, 1);
 
-  rv = mDirProvider.SetProfile(mProfD, mProfLD);
-  NS_ENSURE_SUCCESS(rv, 1);
-
-  //////////////////////// NOW WE HAVE A PROFILE ////////////////////////
-
-  mozilla::Telemetry::SetProfileDir(mProfD);
-
-  if (mAppData->flags & NS_XRE_ENABLE_CRASH_REPORTER)
-    MakeOrSetMinidumpPath(mProfD);
-
-  CrashReporter::SetProfileDirectory(mProfD);
-
-#ifdef MOZ_ASAN_REPORTER
-  // In ASan reporter builds, we need to set ASan's log_path as early as
-  // possible, so it dumps its errors into files there instead of using
-  // the default stderr location. Since this is crucial for ASan reporter
-  // to work at all (and we don't want people to use a non-functional
-  // ASan reporter build), all failures while setting log_path are fatal.
-  setASanReporterPath(mProfD);
-
-  // Export to env for child processes
-  SaveFileToEnv("ASAN_REPORTER_PATH", mProfD);
-#endif
-
   nsAutoCString version;
   BuildVersion(version);
 
@@ -3891,9 +4049,45 @@ int XREMain::XRE_mainStartup(bool* aExitFlag) {
   }
 
   bool cachesOK;
-  bool versionOK =
-      CheckCompatibility(mProfD, version, osABI, mDirProvider.GetGREDir(),
-                         mAppData->directory, flagFile, &cachesOK);
+  bool isDowngrade;
+  bool versionOK = CheckCompatibility(
+      mProfD, version, osABI, mDirProvider.GetGREDir(), mAppData->directory,
+      flagFile, &cachesOK, &isDowngrade);
+
+#ifdef MOZ_BLOCK_PROFILE_DOWNGRADE
+  if (isDowngrade && !CheckArg("allow-downgrade")) {
+    rv = CheckDowngrade(mProfD, mProfLD, mProfileName, mNativeApp, mProfileSvc);
+    if (rv == NS_ERROR_LAUNCHED_CHILD_PROCESS || rv == NS_ERROR_ABORT) {
+      *aExitFlag = true;
+      return 0;
+    }
+  }
+#endif
+
+  rv = mDirProvider.SetProfile(mProfD, mProfLD);
+  NS_ENSURE_SUCCESS(rv, 1);
+
+  //////////////////////// NOW WE HAVE A PROFILE ////////////////////////
+
+  mozilla::Telemetry::SetProfileDir(mProfD);
+
+  if (mAppData->flags & NS_XRE_ENABLE_CRASH_REPORTER) {
+    MakeOrSetMinidumpPath(mProfD);
+  }
+
+  CrashReporter::SetProfileDirectory(mProfD);
+
+#ifdef MOZ_ASAN_REPORTER
+  // In ASan reporter builds, we need to set ASan's log_path as early as
+  // possible, so it dumps its errors into files there instead of using
+  // the default stderr location. Since this is crucial for ASan reporter
+  // to work at all (and we don't want people to use a non-functional
+  // ASan reporter build), all failures while setting log_path are fatal.
+  setASanReporterPath(mProfD);
+
+  // Export to env for child processes
+  SaveFileToEnv("ASAN_REPORTER_PATH", mProfD);
+#endif
 
   bool lastStartupWasCrash = CheckLastStartupWasCrash().unwrapOr(false);