In Firefox when you click on a file to download / view the browser comes up with a prompt which allows you to either open the file with a specified program or save it to disk. You are also given the option to "Do this automatically for files like this from now on". Once you check that option Firefox automatically uses the same action for the same file type in future. Sometimes you check the box by mistake or down the road you may not want the automatic handling. Here's how you can undo this preference.
