I believe the problem with firefox is for newer versions. The community here, seems to have come the consensus that running an older version is the way to go, for now.
Based on the "Welcome" application in Ubuntu MATE 16.04 LTS for the Raspberry Pi, I thought that installing google-chrome did not work and that installing 'chromium' is the accepted workaround and although, I have not tried it, I believe you can just:
sudo apt-get install chromium-browser