Really, the domestic politics of the McChrystal firing are a no brainer.
Obama's avowed opponents, most of them anyway, consciously provided the president with cover from war supporters. War opponents, while not rejoicing at the maintenance of the strategy, will find it comforting that Obama knew enough to know that he had to make a change. Democratic political strategists are going to be gleeful. They're going to be gleeful because the president acted quickly and decisively, without much dithering or delay, and acted with virtually unimpeachable reasoning.
To put it crudely, POTUS proved he has a pair. Having spent a good deal of time talking with administration officials, I believe them when they say that politics are of secondary importance here; I also know that, in the back of their minds, they certainly didn't feel constrained by the domestic political environment in the same way they did when the strategy review began.
No question: the very public firing of a top general by the most powerful man in the world is a major world event. It is the type of direct action that can refocus millions of minds. So the opportunity for Obama to pivot away from the (alleged) fantasyland and bickering that had characterized Afghanistan policy (and maybe even his effort to sell the country on the idea of economic progress and the benefits of health care reform) could not be greater.
Any time the commander in chief can be the commander in chief, well, the politics are good for the president.