imgs/selenium: use firefox-esr instead of bleeding edge

This switches to stable Firefox. There are some issues with bleeding
edge Firefox and we should ensure that we work with stable version more
than with newest one.

imgs/selenium.sh

@@ -24,7 +24,7 @@ rc-update add xvfb
 rc-update add x11vnc
 
 # Firefox
-apk add firefox gcompat tar
+apk add firefox-esr gcompat tar
 wget "$GECKODRIVER_URL" -O - | tar -xzf - -C /usr/bin geckodriver
 rc-update add geckodriver