FirefoxIn 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.

1. Go to Preference option (normally under Edit menu) and click on Content tab.
2. In the Content tab look for File Types section and click on Manage button.

Here you can see all the file types and associated actions. You can click on a File Type and then choose either to change the current default action or remove the action. In change action popup you can either choose, as default action, to automatically save the file or choose a program to open the file type with.

The remove action removes the associated default action.

Sometimes you may find the remove action disabled. To enable it select change action and then after noting down the current action, change the current action to Save them on my computer. Click ok to go back to Download Actions popup. Now you will find the Remove Action button enabled.

Note: To switch back to the previous default action, click on Change Action again and change the action to the setting you saved earlier.