If you didn't jailbreak beforehand and capture the SHSH blob, no, you can't. So for the vast majority of people who upgraded to iOS 6 there is no going back.

http://blog.iphone-dev.org/

That's the cost of being inside a walled garden. You don't even have the freedom to install previous firmwares on your device.

But since the browser is part of a firmware upgrade it would be a terrible idea anyway, a new security bug has been found on mobile safari recently, you don't want to stay on older fimrwares if you can help it. Google maps, or a secure browser ? Pick one.