"Given that Samsung is involved, this browser will be focused on Android and ARM-based SoCs"
More like it will be focused on Linux kernel based platforms and not just Android. Seems like Samsung's partnership with Intel for Tizen OS and who else better to partner with then Mozilla for the browser.
so is a gun, until you put a finger on the trigger. The only ways this could be "safe by default" is by obscurity (probably) or by creating a randomly generated hypervisor and running the browser within a shelled off mini OS (very unlikely.)
Although hypervisor attacks have been proven in concept and in labs, none of them have been found in the wild (to my knowledge)
It's not a surprise to see Samsung is trying to move to another phone OS even though they are the leading Android market now. When Google start to create competing product with Motorola which they owned, it is sure that they will give more privilege to their own product rather then given their best support to Samsung on android front. That's why there were news that Samsung is trying to develop their own handphone OS when Google acquired Motorola.
Only time will tell on what the big G will do and it is the right move for Samsung to start planning for Plan B.