This week's Haftorah comes from Yehezkel, a section dealing with national purity. The connection between this Haftorah and Parshat Parah is the way that Yehezkel describes the purification process, but the Haftorah itself is describing more than just purification.
The Haftorah divides itself neatly into three parts 1: the purification process 2: the goal and the way it is to be attained 3: God will purify Israel even though it's failed so far in the task.
A great deal is written on the first two segments so I'll deal with part three. This section is describing a forced purification one that is not supposed to happen unless there is a great need. This prophecy is describing a time when the Jewish people are so impure that God has to take it upon himself to fix the situation. Sometimes you find parents fixing the mistakes of their children when they've fallen to a point that they can't pull themselves out of. This is not one of those times.
Not for your sakes do I do it, says my Lord, the God who revealed His Love in dealing out justice, be it known unto you, be ashamed and confounded for your ways, the children of Israel. (36:32)God here is telling b'nai Yisrael that "I'm going to fix everything you did wrong, I'm going to purify you, rebuild your cities, replant your land; but the nations will know that I did it, not for you, but for myself for you disgraced my name". This action of God's is not selfish, but rather Israel forgot about marit ayin how their actions are perceived. B'nai Israel forgot what ohr l'goyim means, how they're supposed to be the "priests of mankind" (as Rav Hirsch describes them). Sometimes it is necessary to take things into your own hands and fix it - not often, but on occasion - in such a scenario the teacher/parent should explain his actions and that is what God does here in the previous sections. How purity works, what should be, and why I'm in this one situation not doing that.
Then the nations that are left round about you will recognize that I, God have built up the ruined places and have planted the deserted ones, I, God have spoken it and I will do it.(36:36)