By default you can't. But if you're brave you can edit a config file in your profile directory. I posted about it once before here:
http://www.linuxquestions.org/questi...1/#post4383441
Be aware that this depends on the web server reporting the correct mime-type for the file. Misconfigured servers occasionally send files as the wrong type and your browser just doesn't know what to do with them, in which case it usually pops up a save dialog.
Finally, for certain file formats that the browser can handle natively, but which the browser wants to save due to the way the server sends the file (and a couple of long-standing firefox bugs), you can use this convenient extension to feed it back into the browser for display.
http://spasche.net/openinbrowser/