Removed Firefox from Admin CI runs

refs https://ghost.slack.com/archives/C02G9E68C/p1677753889082979

- Firefox tests have historically been flaky in CI and a real
  distraction when developing
- Firefox is, unfortunately :(, not a widely used browser at only 2.9%
  of global market share
- we've not had any Firefox-specific bugs that were detected in CI for
  a very long time, so it doesn't add anything anyway

.github/workflows/test.yml

@@ -38,10 +38,7 @@ jobs:
   admin-tests:
     runs-on: linux
     if: github.event_name == 'push' || (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'renovate/'))
-    strategy:
-      matrix:
-        browser: ["Firefox", "Chrome"]
-    name: Admin - ${{ matrix.browser }}
+    name: Admin - Chrome
     env:
       MOZ_HEADLESS: 1
       JOBS: 1
@@ -56,13 +53,12 @@ jobs:
       - run: yarn
       - run: yarn workspace ghost-admin run test
         env:
-          BROWSER: ${{ matrix.browser }}
+          BROWSER: Chrome
 
       # Merge coverage reports and upload
       - run: yarn ember coverage-merge
         working-directory: ghost/admin
       - uses: actions/upload-artifact@v3
-        if: matrix.browser == 'Chrome'
         with:
           name: coverage
           path: ghost/*/coverage/cobertura-coverage.xml