diff --git a/pkgs/applications/networking/browsers/firefox/common.nix b/pkgs/applications/networking/browsers/firefox/common.nix
index 3f21274f81a7..b51cc38cbc0d 100644
--- a/pkgs/applications/networking/browsers/firefox/common.nix
+++ b/pkgs/applications/networking/browsers/firefox/common.nix
@@ -328,6 +328,12 @@ buildStdenv.mkDerivation {
       # Fixed on Firefox 140
       ./build-fix-RELRHACK_LINKER-setting-when-linker-name-i.patch
     ]
+    ++ lib.optionals (lib.versionOlder version "138") [
+      # https://bugzilla.mozilla.org/show_bug.cgi?id=1941479
+      # https://phabricator.services.mozilla.com/D240572
+      # Fixed on Firefox 138
+      ./firefox-cannot-find-type-Allocator.patch
+    ]
     ++ extraPatches;
 
   postPatch =
diff --git a/pkgs/applications/networking/browsers/firefox/firefox-cannot-find-type-Allocator.patch b/pkgs/applications/networking/browsers/firefox/firefox-cannot-find-type-Allocator.patch
new file mode 100644
index 000000000000..5ab5401e1c43
--- /dev/null
+++ b/pkgs/applications/networking/browsers/firefox/firefox-cannot-find-type-Allocator.patch
@@ -0,0 +1,26 @@
+From 518049ce568d01413eeda304e8e9c341ab8849f6 Mon Sep 17 00:00:00 2001
+From: Mike Hommey <mh+mozilla@glandium.org>
+Date: Thu, 6 Mar 2025 09:36:10 +0000
+Subject: [PATCH] Bug 1941479 - Mark mozilla::SmallPointerArray_Element as
+ opaque. r=emilio
+
+Differential Revision: https://phabricator.services.mozilla.com/D240572
+---
+ layout/style/ServoBindings.toml | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/layout/style/ServoBindings.toml b/layout/style/ServoBindings.toml
+index 86c6c3026ce7..2b9a34a81a0f 100644
+--- a/layout/style/ServoBindings.toml
++++ b/layout/style/ServoBindings.toml
+@@ -301,6 +301,7 @@ opaque-types = [
+     "mozilla::dom::Touch",
+     "mozilla::dom::Sequence",
+     "mozilla::SmallPointerArray",
++    "mozilla::SmallPointerArray_Element",
+     "mozilla::dom::Optional",
+     "mozilla::dom::OwningNodeOrString_Value",
+     "mozilla::dom::Nullable",
+-- 
+2.49.0
+