diff --git a/libraries/source/spidermonkey/build.sh b/libraries/source/spidermonkey/build.sh
index 62716f58cc..8e00a4214a 100755
--- a/libraries/source/spidermonkey/build.sh
+++ b/libraries/source/spidermonkey/build.sh
@@ -9,7 +9,7 @@ cd "$(dirname "$0")"
 PV=128.13.0
 FOLDER="mozjs-${PV}"
 # If same-version changes are needed, increment this.
-LIB_VERSION="${PV}+wfg4"
+LIB_VERSION="${PV}+wfg5"
 LIB_NAME="mozjs128"
 
 build_archive()
@@ -100,6 +100,12 @@ rm -Rf "${FOLDER}"
 	# Do not let mach install tools in the user's home, create a disposable folder
 	export MOZBUILD_STATE_PATH="${MOZBUILD_STATE_PATH:=$(pwd)/mozbuild-state}"
 
+	if [ "${OS}" != "Darwin" ]; then
+		# Make thin archives to save disk space
+		# The AR_FLAGS customization point is provided and honored by mach
+		export AR_FLAGS="--thin"
+	fi
+
 	if [ -n "$PROFILE" ]; then
 		CONF_OPTS="$CONF_OPTS
 			--enable-profiling
diff --git a/libraries/source/spidermonkey/patches/UpstreamCustomARFlags.patch b/libraries/source/spidermonkey/patches/UpstreamCustomARFlags.patch
new file mode 100644
index 0000000000..d3b52045a4
--- /dev/null
+++ b/libraries/source/spidermonkey/patches/UpstreamCustomARFlags.patch
@@ -0,0 +1,61 @@
+
+# HG changeset patch
+# User serge-sans-paille <sguelton@mozilla.com>
+# Date 1768903043 0
+# Node ID a924182c4e55e19b1b8dbd5904e8f9ca99e79b98
+# Parent  cd5449e09b6cd3e3b0bb1f2d6d8a8048e8883535
+Bug 2008085 - Provide AR_FLAGS customization point r=glandium
+
+In a similar way to other customization flags.
+
+That way users can specify `--thin` if they want to to reduce disk
+pressure.
+
+Differential Revision: https://phabricator.services.mozilla.com/D278841
+
+
+diff --git a/build/moz.configure/toolchain.configure b/build/moz.configure/toolchain.configure
+--- a/build/moz.configure/toolchain.configure
++++ b/build/moz.configure/toolchain.configure
+@@ -4,6 +4,13 @@
+ # License, v. 2.0. If a copy of the MPL was not distributed with this
+ # file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ 
++option(
++    env="AR_FLAGS",
++    help="Extra flags for creating static archives",
++    nargs=1,
++    default="",
++)
++
+ # Code optimization
+ # ==============================================================
+ 
+@@ -3346,12 +3353,14 @@
+     if not toolchain_prefix:
+         toolchain_prefix = dependable(None)
+ 
+-    @depends(toolchain_prefix, c_compiler)
+-    def ar_config(toolchain_prefix, c_compiler):
++    @depends(toolchain_prefix, c_compiler, "AR_FLAGS")
++    @imports(_from="mozbuild.shellutil", _import="split")
++    def ar_config(toolchain_prefix, c_compiler, env_flags):
++        extra_ar_flags = tuple(split(env_flags[0]))
+         if c_compiler.type == "clang-cl":
+             return namespace(
+                 names=("llvm-lib",),
+-                flags=("-llvmlibthin", "-out:$@"),
++                flags=("-llvmlibthin", "-out:$@") + extra_ar_flags,
+             )
+ 
+         names = tuple("%s%s" % (p, "ar") for p in (toolchain_prefix or ()) + ("",))
+@@ -3368,7 +3377,7 @@
+ 
+         return namespace(
+             names=names,
+-            flags=("crs", "$@"),
++            flags=("crs",) + extra_ar_flags + ("$@",),
+         )
+ 
+     return ar_config
+
diff --git a/libraries/source/spidermonkey/patches/patch.sh b/libraries/source/spidermonkey/patches/patch.sh
index 04bbe2bf4e..8ea5bccf68 100755
--- a/libraries/source/spidermonkey/patches/patch.sh
+++ b/libraries/source/spidermonkey/patches/patch.sh
@@ -47,3 +47,6 @@ patch -p1 <"${PATCHES}"/SuppressDanglingPointerWarning.patch
 
 # Fix building with python 3.14
 patch -p1 <"${PATCHES}"/FixPython3_14.diff
+
+# Cherry-pick an upstream commit to allow passing custom flags to ar
+patch -p1 <"${PATCHES}"/UpstreamCustomARFlags.patch