diff --git a/firefox.spec b/firefox.spec
index 9d407b3..1486383 100644
--- a/firefox.spec
+++ b/firefox.spec
@@ -715,10 +715,34 @@ echo "ac_add_options MOZ_PGO=1" >> .mozconfig
 export CCACHE_DISABLE=1
 %endif
 
+%if 0%{?fedora} > 35
 # Require 2 GB of RAM per CPU core
 %constrain_build -m 2048
-
 echo "mk_add_options MOZ_MAKE_FLAGS=\"-j%{_smp_build_ncpus}\"" >> .mozconfig
+%else
+# F35 doesn't have %%constrain_build
+MOZ_SMP_FLAGS=-j1
+# On x86_64 architectures, Mozilla can build up to 4 jobs at once in parallel,
+# however builds tend to fail on other arches when building in parallel.
+%ifarch %{ix86} s390x %{arm} aarch64
+[ -z "$RPM_BUILD_NCPUS" ] && \
+     RPM_BUILD_NCPUS="`/usr/bin/getconf _NPROCESSORS_ONLN`"
+[ "$RPM_BUILD_NCPUS" -ge 2 ] && MOZ_SMP_FLAGS=-j2
+%endif
+%ifarch x86_64 ppc ppc64 ppc64le
+[ -z "$RPM_BUILD_NCPUS" ] && \
+     RPM_BUILD_NCPUS="`/usr/bin/getconf _NPROCESSORS_ONLN`"
+[ "$RPM_BUILD_NCPUS" -ge 2 ] && MOZ_SMP_FLAGS=-j2
+[ "$RPM_BUILD_NCPUS" -ge 4 ] && MOZ_SMP_FLAGS=-j4
+[ "$RPM_BUILD_NCPUS" -ge 8 ] && MOZ_SMP_FLAGS=-j8
+[ "$RPM_BUILD_NCPUS" -ge 16 ] && MOZ_SMP_FLAGS=-j16
+[ "$RPM_BUILD_NCPUS" -ge 24 ] && MOZ_SMP_FLAGS=-j24
+[ "$RPM_BUILD_NCPUS" -ge 32 ] && MOZ_SMP_FLAGS=-j32
+[ "$RPM_BUILD_NCPUS" -ge 64 ] && MOZ_SMP_FLAGS=-j64
+%endif
+echo "mk_add_options MOZ_MAKE_FLAGS=\"$MOZ_SMP_FLAGS\"" >> .mozconfig
+%endif
+
 echo "mk_add_options MOZ_SERVICES_SYNC=1" >> .mozconfig
 echo "export STRIP=/bin/true" >> .mozconfig
 #export MACH_USE_SYSTEM_PYTHON=1