Oh my god, I have the same problem. But some details I must add:
Launcher Version: Launcher Build 253
Operating System: Mac OS X 10.8.4
Java Version: 1.6.0_51
Antivirus Version: [not sure]
Description: Well, instead of 2 windows opening, I open a modpack, then it loads up, connects to minecraft.net and stuff, then once it opens, the screen is white for a while, and then it just disappears. It doesn't reach the Mojang screen, and there are no crash reports! I tried this with Tekkit Main, Tekkit Classic, Yogbox, and Hexxit, and none of them worked!