DEV: Also run ember tests in Firefox

DEV: Also run ember tests in Firefox

diff --git a/image/discourse_dev/Dockerfile b/image/discourse_dev/Dockerfile
index b215ed6..e889974 100644
--- a/image/discourse_dev/Dockerfile
+++ b/image/discourse_dev/Dockerfile
@@ -41,8 +41,6 @@ ADD ensure-database /etc/runit/1.d/ensure-database
 
 RUN wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | sudo apt-key add - &&\
     echo "deb [arch=amd64] http://dl.google.com/linux/chrome/deb/ stable main" >> /etc/apt/sources.list.d/google.list &&\
-    curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add - &&\
-    echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list &&\
     apt update &&\
-    apt install -y google-chrome-stable yarn nodejs &&\
+    apt install -y google-chrome-stable firefox-esr &&\
     npm install -g eslint babel-eslint

GitHub sha: d457b6b9a8d57db4d474d6a7d5b636d5773577e8

This commit appears in #560 which was approved by eviltrout. It was merged by Falco.