diff --git a/extra/js91/PKGBUILD b/extra/js91/PKGBUILD
new file mode 100644
index 000000000..1c20e3a30
--- /dev/null
+++ b/extra/js91/PKGBUILD
@@ -0,0 +1,130 @@
+# Maintainer: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
+
+# ALARM: Kevin Mihelich <kevin@archlinuxarm.org>
+#  - configure with --disable-lto, --disable-rust-simd
+
+pkgname=js91
+pkgver=91.5.0
+pkgrel=1
+pkgdesc="JavaScript interpreter and libraries - Version 91"
+arch=(x86_64)
+url="https://spidermonkey.dev/"
+license=(MPL)
+depends=(gcc-libs readline zlib sh)
+makedepends=(zip autoconf2.13 python-setuptools python-psutil rust llvm clang lld)
+checkdepends=(mercurial git)
+options=(!lto)
+_relver=${pkgver}esr
+source=(https://archive.mozilla.org/pub/firefox/releases/$_relver/source/firefox-$_relver.source.tar.xz{,.asc})
+sha256sums=('f45cd9c96227e3e6eabe37962ce924b7a7ca86b6c191326c1bab18e082b4c813'
+            'SKIP')
+validpgpkeys=('14F26682D0916CDD81E37B6D61B7B526D98F0353') # Mozilla Software Releases <release@mozilla.com>
+
+# Make sure the duplication between bin and lib is found
+COMPRESSZST+=(--long)
+
+prepare() {
+  mkdir mozbuild
+  cd firefox-$pkgver
+
+  cat >../mozconfig <<END
+ac_add_options --enable-application=js
+mk_add_options MOZ_OBJDIR=${PWD@Q}/obj
+
+ac_add_options --prefix=/usr
+ac_add_options --enable-release
+ac_add_options --disable-lto
+ac_add_options --enable-hardening
+ac_add_options --enable-optimize
+ac_add_options --disable-rust-simd
+ac_add_options --enable-linker=lld
+ac_add_options --disable-bootstrap
+ac_add_options --disable-debug
+ac_add_options --disable-debug-symbols
+ac_add_options --disable-jemalloc
+ac_add_options --disable-strip
+
+# System libraries
+ac_add_options --with-system-zlib
+ac_add_options --without-system-icu
+
+# Features
+ac_add_options --enable-readline
+ac_add_options --enable-shared-js
+ac_add_options --enable-tests
+ac_add_options --with-intl-api
+END
+}
+
+build() {
+  cd firefox-$pkgver
+
+  export MOZ_NOSPAM=1
+  export MOZBUILD_STATE_PATH="$srcdir/mozbuild"
+  export MACH_USE_SYSTEM_PYTHON=1
+
+  # Do 3-tier PGO
+  echo "Building instrumented JS..."
+  cat >.mozconfig ../mozconfig - <<END
+ac_add_options --enable-profile-generate=cross
+END
+  ./mach build
+
+  echo "Profiling instrumented JS..."
+  (
+    local js="$PWD/obj/dist/bin/js"
+    export LLVM_PROFILE_FILE="$PWD/js-%p-%m.profraw"
+
+    cd js/src/octane
+    "$js" run.js
+
+    cd ../../../third_party/webkit/PerformanceTests/ARES-6
+    "$js" cli.js
+
+    cd ../SunSpider/sunspider-0.9.1
+    "$js" sunspider-standalone-driver.js
+  )
+
+  llvm-profdata merge -o merged.profdata *.profraw
+
+  stat -c "Profile data found (%s bytes)" merged.profdata
+  test -s merged.profdata
+
+  echo "Removing instrumented JS..."
+  ./mach clobber
+
+  echo "Building optimized JS..."
+  cat >.mozconfig ../mozconfig - <<END
+ac_add_options --enable-profile-use=cross
+ac_add_options --with-pgo-profile-path=${PWD@Q}/merged.profdata
+END
+  ./mach build
+}
+
+check() {
+  local jstests_extra_args=(
+    --format=none
+    --exclude-random
+    --wpt=disabled
+  ) jittest_extra_args=(
+    --format=none
+    --timeout 300
+  ) jittest_test_args=(
+    basic
+  )
+
+  cd firefox-$pkgver/obj
+  make -C js/src check-jstests check-jit-test \
+    JSTESTS_EXTRA_ARGS="${jstests_extra_args[*]}" \
+    JITTEST_EXTRA_ARGS="${jittest_extra_args[*]}" \
+    JITTEST_TEST_ARGS="${jittest_test_args[*]}"
+}
+
+package() {
+  cd firefox-$pkgver/obj
+  make DESTDIR="$pkgdir" install
+  rm "$pkgdir"/usr/lib/*.ajs
+  find "$pkgdir"/usr/{lib/pkgconfig,include} -type f -exec chmod -c a-x {} +
+}
+
+# vim:set sw=2 et: