DEV: Run Ember CLI tests in Firefox evergreen too

DEV: Run Ember CLI tests in Firefox evergreen too

diff --git a/app/assets/javascripts/discourse/testem.js b/app/assets/javascripts/discourse/testem.js
index 1bc59b3..1934f7c 100644
--- a/app/assets/javascripts/discourse/testem.js
+++ b/app/assets/javascripts/discourse/testem.js
@@ -1,9 +1,9 @@
 module.exports = {
   test_page: "tests/index.html?hidepassed",
   disable_watching: true,
-  launch_in_ci: ["Chrome", "Firefox"],
+  launch_in_ci: ["Chrome", "Firefox", "Headless Firefox"], // Firefox is old ESR version, Headless Firefox is up-to-date evergreen version
   launch_in_dev: ["Chrome"],
-  parallel: -1, // run Firefox and Chrome in parallel
+  parallel: -1, // run all in parallel
   browser_args: {
     Chrome: [
       // --no-sandbox is needed when running Chrome inside a container
@@ -16,5 +16,9 @@ module.exports = {
       "--window-size=1440,900",
     ].filter(Boolean),
     Firefox: ["-headless", "--width=1440", "--height=900"],
+    "Headless Firefox": ["--width=1440", "--height=900"],
+  },
+  browser_paths: {
+    "Headless Firefox": "/opt/firefox-evergreen/firefox",
   },
 };

GitHub sha: 6a186afaec153e4a7804190b99c80aa1ed6d73bf

Should there be a headless flag here too?