Who's "we"? I'm not firing anyone, MSNBC fired him. They could easily have chosen not to. It was all up to them; there was no force behind it.
Sure, there was some boycott that Al Sharpton came up with, but a boycott is only effective in one of two conditions:
1) There is enough in the boycott for the company to risk losing massive amounts of profit.
2) The boycott seems to be far more effective than it really is.
It seems to be 2, in this case. The company (and more specifically their investors) perceived a feint, took it as a direct and deadly attack, responded, and Don Imus got fired for it.
Though I agree with Randfan that emotions were overly charged in this situation, and Al Sharpton attempted to charge them even more. Regardless, if I put blame anywhere, it's on MSNBC and their investors, notably the latter. They chickened out and weren't willing to stand behind their man.
However, if the people that hired any of those people that you just mentioned decided to fire them, then I support their right to.