diff --git a/etc/ci/buildbot_steps.yml b/etc/ci/buildbot_steps.yml
index a08fb2ad5b9b..5a1a2ea88e8b 100644
--- a/etc/ci/buildbot_steps.yml
+++ b/etc/ci/buildbot_steps.yml
@@ -7,7 +7,7 @@ env:
 mac-rel-wpt1:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig CC=gcc-5 CXX=g++-5 ./mach build --release
   - ./mach run -r -o output.png
   - ./mach test-wpt-failure
   - ./mach test-wpt --release --processes 4 --total-chunks 6 --this-chunk 1 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
@@ -19,7 +19,7 @@ mac-rel-wpt1:
 mac-rel-wpt2:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig CC=gcc-5 CXX=g++-5 ./mach build --release
   - ./mach test-wpt --release --processes 4 --total-chunks 6 --this-chunk 2 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build-geckolib --release
@@ -27,14 +27,14 @@ mac-rel-wpt2:
 mac-rel-wpt3:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig CC=gcc-5 CXX=g++-5 ./mach build --release
   - ./mach test-wpt --release --processes 4 --total-chunks 6 --this-chunk 3 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
 
 mac-rel-wpt4:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig CC=gcc-5 CXX=g++-5 ./mach build --release
   - ./mach test-wpt --release --processes 4 --total-chunks 6 --this-chunk 4 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
   - ./mach test-wpt --release --pref dom.servoparser.async_html_tokenizer.enabled --processes=8 --log-raw test-async-parsing.log --log-errorsummary async-parsing-errorsummary.log --always-succeed domparsing html/syntax html/dom/documents html/dom/dynamic-markup-insertion
@@ -43,8 +43,8 @@ mac-rel-wpt4:
 mac-dev-unit:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --dev
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach test-unit
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig CC=gcc-5 CXX=g++-5 ./mach build --dev
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig CC=gcc-5 CXX=g++-5 ./mach test-unit
   - ./mach package --dev
   - ./mach build-geckolib
   - bash ./etc/ci/lockfile_changed.sh
@@ -53,7 +53,7 @@ mac-dev-unit:
 mac-rel-css1:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig CC=gcc-5 CXX=g++-5 ./mach build --release
   - ./mach test-wpt --release --processes 4 --total-chunks 6 --this-chunk 5 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
   - bash ./etc/ci/lockfile_changed.sh
@@ -62,14 +62,14 @@ mac-rel-css1:
 mac-rel-css2:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig CC=gcc-5 CXX=g++-5 ./mach build --release
   - ./mach test-wpt --release --processes 4 --total-chunks 6 --this-chunk 6 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
 
 mac-nightly:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig CC=gcc-5 CXX=g++-5 ./mach build --release
   - ./mach package --release
   - ./mach upload-nightly mac
   - ./etc/ci/update-wpt-checkout fetch-and-update-expectations
@@ -94,7 +94,7 @@ linux-rel-nogate:
 mac-rel-intermittent:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
-  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
+  - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig CC=gcc-5 CXX=g++-5 ./mach build --release
   - ./etc/ci/check_intermittents.sh --log-raw intermittents.log
 
 linux-dev: