diff --git a/browser/app/nsBrowserApp.cpp b/browser/app/nsBrowserApp.cpp
--- a/browser/app/nsBrowserApp.cpp
+++ b/browser/app/nsBrowserApp.cpp
@@ -154,19 +154,31 @@ static bool IsArg(const char* arg, const
 #endif
 
   return false;
 }
 
 Bootstrap::UniquePtr gBootstrap;
 
 static int do_main(int argc, char* argv[], char* envp[]) {
+  // Allow profile downgrade for Zotero
+  setenv("MOZ_ALLOW_DOWNGRADE", "1", 1);
+  // Don't create dedicated profile (default-esr)
+  setenv("MOZ_LEGACY_PROFILES", "1", 1);
+
   // Allow firefox.exe to launch XULRunner apps via -app <application.ini>
   // Note that -app must be the *first* argument.
-  const char* appDataFile = getenv("XUL_APP_FILE");
+  UniqueFreePtr<char> iniPath = BinaryPath::GetApplicationIni();
+  if (!iniPath) {
+    Output("Couldn't find application.ini.\n");
+    return 255;
+  }
+  char *appDataFile = iniPath.get();
+
+
   if ((!appDataFile || !*appDataFile) && (argc > 1 && IsArg(argv[1], "app"))) {
     if (argc == 2) {
       Output("Incorrect number of arguments passed to -app");
       return 255;
     }
     appDataFile = argv[2];
 
     char appEnv[MAXPATHLEN];
diff --git a/python/mozbuild/mozbuild/action/unpack_dmg.py b/python/mozbuild/mozbuild/action/unpack_dmg.py
--- a/python/mozbuild/mozbuild/action/unpack_dmg.py
+++ b/python/mozbuild/mozbuild/action/unpack_dmg.py
@@ -34,18 +34,18 @@ def main(args):
     options = parser.parse_args(args)
 
     dmg_tool = bootstrap_toolchain("dmg/dmg")
     hfs_tool = bootstrap_toolchain("dmg/hfsplus")
 
     dmg.extract_dmg(
         dmgfile=Path(options.dmgfile),
         output=Path(options.outpath),
-        dmg_tool=Path(dmg_tool),
-        hfs_tool=Path(hfs_tool),
+        dmg_tool=_path_or_none(dmg_tool),
+        hfs_tool=_path_or_none(hfs_tool),
         dsstore=_path_or_none(options.dsstore),
         background=_path_or_none(options.background),
         icon=_path_or_none(options.icon),
     )
     return 0
 
 
 if __name__ == "__main__":
diff --git a/xpcom/build/BinaryPath.h b/xpcom/build/BinaryPath.h
--- a/xpcom/build/BinaryPath.h
+++ b/xpcom/build/BinaryPath.h
@@ -128,16 +128,56 @@ class BinaryPath {
     } else {
       rv = NS_ERROR_FAILURE;
     }
 
     CFRelease(executableURL);
     return rv;
   }
 
+  static nsresult GetApplicationIni(char aResult[MAXPATHLEN])
+  {
+    // Works even if we're not bundled.
+    CFBundleRef appBundle = CFBundleGetMainBundle();
+    if (!appBundle) {
+      return NS_ERROR_FAILURE;
+    }
+
+    CFURLRef iniURL = CFBundleCopyResourceURL(appBundle, CFSTR("application.ini"),
+                                              NULL, CFSTR("app"));
+    if (!iniURL) {
+      return NS_ERROR_FAILURE;
+    }
+
+    nsresult rv;
+    if (CFURLGetFileSystemRepresentation(iniURL, false, (UInt8*)aResult,
+                                         MAXPATHLEN)) {
+      // Sanitize path in case the app was launched from Terminal via
+      // './firefox' for example.
+      size_t readPos = 0;
+      size_t writePos = 0;
+      while (aResult[readPos] != '\0') {
+        if (aResult[readPos] == '.' && aResult[readPos + 1] == '/') {
+          readPos += 2;
+        } else {
+          aResult[writePos] = aResult[readPos];
+          readPos++;
+          writePos++;
+        }
+      }
+      aResult[writePos] = '\0';
+      rv = NS_OK;
+    } else {
+      rv = NS_ERROR_FAILURE;
+    }
+
+    CFRelease(iniURL);
+    return rv;
+  }
+
 #elif defined(ANDROID)
   static nsresult Get(char aResult[MAXPATHLEN]) {
     // On Android, we use the MOZ_ANDROID_LIBDIR variable that is set by the
     // Java bootstrap code.
     const char* libDir = getenv("MOZ_ANDROID_LIBDIR");
     if (!libDir) {
       return NS_ERROR_FAILURE;
     }
@@ -283,16 +323,29 @@ class BinaryPath {
     if (NS_FAILED(Get(path))) {
       return nullptr;
     }
     UniqueFreePtr<char> result;
     result.reset(strdup(path));
     return result;
   }
 
+#if defined(XP_MACOSX)
+  static UniqueFreePtr<char> GetApplicationIni()
+  {
+    char path[MAXPATHLEN];
+    if (NS_FAILED(GetApplicationIni(path))) {
+      return nullptr;
+    }
+    UniqueFreePtr<char> result;
+    result.reset(strdup(path));
+    return result;
+  }
+#endif
+
 #ifdef MOZILLA_INTERNAL_API
   static nsresult GetFile(nsIFile** aResult) {
     nsCOMPtr<nsIFile> lf;
 #  ifdef XP_WIN
     wchar_t exePath[MAXPATHLEN];
     nsresult rv = GetW(exePath);
 #  else
     char exePath[MAXPATHLEN];