diff --git a/extra/xulrunner/PKGBUILD b/extra/xulrunner/PKGBUILD
index e51f9ad7f..33e4a6549 100644
--- a/extra/xulrunner/PKGBUILD
+++ b/extra/xulrunner/PKGBUILD
@@ -8,7 +8,7 @@
 highmem=1
 
 pkgname=xulrunner
-pkgver=19.0.2
+pkgver=20.0
 pkgrel=1
 pkgdesc="Mozilla Runtime Environment"
 arch=('i686' 'x86_64')
@@ -23,7 +23,7 @@ source=(ftp://ftp.mozilla.org/pub/mozilla.org/xulrunner/releases/$pkgver/source/
 options=('!emptydirs')
 replaces=('xulrunner-oss')
 
-build() {
+prepare() {
   cd "$srcdir/mozilla-release"
   cp "$srcdir/mozconfig" .mozconfig
 
@@ -35,8 +35,16 @@ build() {
   # Workaround taken from chromium PKGBUILD
   mkdir "$srcdir/python2-path"
   ln -s /usr/bin/python2 "$srcdir/python2-path/python"
-  export PATH="$srcdir/python2-path:$PATH"
 
+  # configure script misdetects the preprocessor without an optimization level
+  # https://bugs.archlinux.org/task/34644
+  sed -i '/ac_cpp=/s/$CPPFLAGS/& -O2/' configure
+}
+
+build() {
+  cd "$srcdir/mozilla-release"
+
+  export PATH="$srcdir/python2-path:$PATH"
   export LDFLAGS="$LDFLAGS -Wl,-rpath,/usr/lib/xulrunner-$pkgver"
   export PYTHON="/usr/bin/python2"
 
@@ -59,7 +67,7 @@ package() {
   sed -i 's|!/usr/bin/env python$|!/usr/bin/env python2|' \
     "$pkgdir"/usr/lib/xulrunner-devel-$pkgver/sdk/bin/{xpt,header,typelib,xpidl}.py
 }
-md5sums=('956f60b12577bf7a42d60ba8e1a73794'
+md5sums=('1b8a1907919eb805f390a05216f2d8d0'
          'ff653a78963f4801df788f4fc0a02867'
          '27271ce647a83906ef7a24605e840d61'
          '52e52f840a49eb1d14be1c0065b03a93')