install firefox (#264)

.github/workflows/ci.yaml

diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index ced8a8b..311bd3d 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -72,6 +72,7 @@
     - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d
       with:
         sdk: ${{ matrix.sdk }}
+    - uses: browser-actions/setup-firefox@latest
     - uses: browser-actions/setup-geckodriver@eb8b0670366f719ca31703766a8cb7e3ea2c56ed
     - id: install
       name: Install dependencies