That is not really something that will happen, the computer will actually save the file at least temporarily on your computer if you open the attachment. What I expect you to be trying to do is * open the file and view the content in Firefox * or open the file directly with some other application What sort of files are you asking about e.g. pictures, Word Documents, or pdf s . I am not yet entirely certain what you re trying to do but you may be interested in the solutions in one or more of these articles. * [[View PDF files in Firefox without downloading them]] *[[Use the Adobe Reader plugin to view or download PDF files]] *[[Set how Firefox handles different types of files]] *[[Change what Firefox does when you click on or download a file]]