Use Firefox headless mode in PolyGerrit tests

Change-Id: Ib9d87d02b77f51fb0742b90c4595a61f829bf178
diff --git a/polygerrit-ui/app/embed_test.sh b/polygerrit-ui/app/embed_test.sh
index d482796..bb08bb0 100755
--- a/polygerrit-ui/app/embed_test.sh
+++ b/polygerrit-ui/app/embed_test.sh
@@ -16,8 +16,7 @@
 
 if [ "${WCT_HEADLESS_MODE:-0}" != "0" ]; then
     CHROME_OPTIONS=[\'start-maximized\',\'headless\',\'disable-gpu\',\'no-sandbox\']
-    # TODO(paladox): Fix Firefox support for headless mode
-    FIREFOX_OPTIONS=[\'\']
+    FIREFOX_OPTIONS=[\'-headless\']
 else
     CHROME_OPTIONS=[\'start-maximized\']
     FIREFOX_OPTIONS=[\'\']
diff --git a/polygerrit-ui/app/wct_test.sh b/polygerrit-ui/app/wct_test.sh
index a8394cd..81dc109 100755
--- a/polygerrit-ui/app/wct_test.sh
+++ b/polygerrit-ui/app/wct_test.sh
@@ -14,8 +14,7 @@
 
 if [ "${WCT_HEADLESS_MODE:-0}" != "0" ]; then
     CHROME_OPTIONS=[\'start-maximized\',\'headless\',\'disable-gpu\',\'no-sandbox\']
-    # TODO(paladox): Fix Firefox support for headless mode
-    FIREFOX_OPTIONS=[\'\']
+    FIREFOX_OPTIONS=[\'-headless\']
 else
     CHROME_OPTIONS=[\'start-maximized\']
     FIREFOX_OPTIONS=[\'\']