从批处理中查找并添加文本文件中的行

I would like to know if anyone can help me with this. I would like to search a text file for a certain line of text, add a new line under the specific line then add text to the new line. I will be using this to edit the firefox.js file to add the line of text to add support for Iprism. It will run on XP and Windows 7 machines.I would like to know if anyone can help me with