You think that makes your brain hurt? The Halos fire superluminally, meaning faster than the speed of light (otherwise it would take at the very least 25,000 years for the arrays pulse to fully cover the galaxy). This means that although from the Arks vantage point you're seeing the galaxy some 260,000 years ago, when the Halos fired, the light would have reached the Ark faster than the speed of light by a commensurate factor. This means that the Halos burst travelled backwards through time meaning Forerunners on the Ark might have seen the Halos fire, maybe even before they actually did (prefiring echoes 343GS talks about), but also means that physical photons from the firing are still travelling to the Ark following its actual firing, meaning some 140,000 years in the future, the firing will be visible AGAIN, from the Arks surface.