Open seamonkey and then go to the edit > preferences menu.
click on the navigator node .On the right side plane you will be able to see a button called set default browser .
Just Click on it and you will be able to set seamonkey as the default web browser .
Setting up seamonkey as the default web browser will make it the default application for handling html documents, htm files and other internet offline content as well.
Also see Very important seamonkey addons.