diff --git a/gnu/packages/gnuzilla.scm b/gnu/packages/gnuzilla.scm
index 15b28f723a..12d3eda67b 100644
--- a/gnu/packages/gnuzilla.scm
+++ b/gnu/packages/gnuzilla.scm
@@ -8,7 +8,7 @@
 ;;; Copyright © 2017, 2023 Clément Lassieur <clement@lassieur.org>
 ;;; Copyright © 2017, 2018 Nikita <nikita@n0.is>
 ;;; Copyright © 2017, 2018, 2020 Tobias Geerinckx-Rice <me@tobias.gr>
-;;; Copyright © 2018, 2020 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2018, 2020, 2025 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2019 Ivan Petkov <ivanppetkov@gmail.com>
 ;;; Copyright © 2020, 2024 Oleg Pykhalov <go.wigust@gmail.com>
 ;;; Copyright © 2020 Jakub Kądziołka <kuba@kadziolka.net>
@@ -142,6 +142,14 @@
          "--with-intl-api")
       #:phases
       #~(modify-phases %standard-phases
+          (add-after 'unpack 'python-3.11-compatibility
+            (lambda _
+              (substitute* '("python/mozbuild/mozpack/files.py"
+                             "python/mozbuild/mozbuild/util.py"
+                             "python/mozbuild/mozbuild/action/process_define_files.py"
+                             "python/mozbuild/mozbuild/backend/base.py"
+                             "python/mozbuild/mozbuild/preprocessor.py")
+                (("\"rU\"") "\"r\""))))
           (add-after 'patch-source-shebangs 'patch-cargo-checksums
             (lambda _
               (let ((null-hash