Replace "/usr/lib/mozilla" (the system-wide directory for extensions and
native manifests) with "$ICECAT_SYSTEM_DIR".

--- a/toolkit/xre/nsXREDirProvider.cpp
+++ b/toolkit/xre/nsXREDirProvider.cpp
@@ -287,24 +287,11 @@
 static nsresult GetSystemParentDirectory(nsIFile** aFile) {
   nsresult rv;
   nsCOMPtr<nsIFile> localDir;
-#  if defined(XP_MACOSX)
-  rv = GetOSXFolderType(kOnSystemDisk, kApplicationSupportFolderType,
-                        getter_AddRefs(localDir));
-  if (NS_SUCCEEDED(rv)) {
-    rv = localDir->AppendNative("Mozilla"_ns);
-  }
-#  else
-  constexpr auto dirname =
-#    ifdef HAVE_USR_LIB64_DIR
-      "/usr/lib64/mozilla"_ns
-#    elif defined(__OpenBSD__) || defined(__FreeBSD__)
-      "/usr/local/lib/mozilla"_ns
-#    else
-      "/usr/lib/mozilla"_ns
-#    endif
-      ;
-  rv = NS_NewNativeLocalFile(dirname, getter_AddRefs(localDir));
-#  endif
+  const char* systemParentDir = getenv("ICECAT_SYSTEM_DIR");
+  if (!systemParentDir || !*systemParentDir) return NS_ERROR_FAILURE;
+
+  rv = NS_NewNativeLocalFile(nsDependentCString(systemParentDir),
+                             getter_AddRefs(localDir));

   if (NS_SUCCEEDED(rv)) {
     localDir.forget(aFile);