If we set up the universe using Newton's mechanics, we can get a (mostly useless) definition of absolute velocity from the big bang itself. If momentum is conserved while energy is not (which it cannot be), absolute velocity is defined from the big bang's initial reference frame.
We can do the same in general relativity for some sets of initial conditions but not others, but there is no simple proof for this because conservation of momentum and conservation of energy are linked in general relativity. In all of the ones for which this works, the absolute velocity is equivalent to the velocity of the cosmic background radiation.
Rockets accelerate by pushing mass out the back. The weak forces resulting from CMB interaction are negligible for any reasonable rocket, therefore if fired in deep space, the reasonable reference frame is the initial frame of the rocket, and there is no change to position of the center of mass of rocket + exhaust. As we should expect from this, engine efficiency is exponential with engine exhaust velocity.
So, the effective answer to your question is "we don't care". The laws of physics from the time of Newton never really cared what the effective frame is. If you take the laws of physics and take the limit* as $c$ goes to infinity, Newton's mechanics drop out again.
*Yes I know taking the limit of a constant makes no mathematical sense. What we're looking for is reintroducing Newton's assumption that the speed of light is too large to matter for anything else.