I think it's too soon for something like this to happen. Ubuntu usually does follow trends and chooses the most used app for one task as default but i doubt currently user ratio between Chromium and Firefox is in favor of Chromium and i doubt solid facts could be made by users why one and not the other... It would probably end up as bad PR stunt on top of recent accusations about less privacy and Dash and developing Mir behind closed doors... because FF is powerful brand in FOSS world and beyond and making it second class citizen in Ubuntu well you decide but in my opinion it would just end up as bad PR stunt ATM with no clear benefits!