Alex Dunn me ajudou, você pode instalar o Firefox 28 ou 50.x com o apt-get, então baixá-lo precisa de algumas dependências, aqui está o arquivo do Docker:
RUN apt-get update
RUN apt-get purge firefox
RUN apt-cache showpkg firefox
RUN apt-get install -y wget libfreetype6 libfontconfig1 libxrender1 libXext6 libXdamage1 libXfixes-dev libXcomposite-dev libasound-dev libdbus-glib-1-dev libgtk2.0-0 libxt6 python-pip
RUN pip install selenium==2.48.0 robotframework==3.0 requests robotframework-requests robotframework-selenium2library==1.8.0 pymysql robotframework-databaselibrary robotframework-excellibrary
RUN pip install requests --upgrade
RUN wget https://ftp.mozilla.org/pub/firefox/releases/45.0/linux-x86_64/en-US/firefox-45.0.tar.bz2
RUN tar -xjf firefox-45.0.tar.bz2
RUN mv firefox /opt/firefox45
RUN ln -s /opt/firefox45/firefox /usr/bin/firefox
RUN ls /opt/firefox45
RUN firefox --version