nsILocalFile:launch
From MDC
[edit] Summary
This method requests that the operating system attempt to open this nsILocalFile
void launch();
[edit] Remarks
launch and reveal are currently (as of 2007/03/14, FX3.0, Gecko 1.9) only implemented for Windows, MacOSX, BeOS and OS/2. On *nix-ish systems like Linux, BSDs, Solaris or similar neither of both functions will perform anything but instead immediately return NS_ERROR_FAILURE.
See bug 67001 for further information. To cover these systems as well you might want to consider the work-around from Firefox Download Manager.
For some "inspiration" see: http://mxr.mozilla.org/seamonkey/source/toolkit/mozapps/downloads/content/downloads.js
To run executable you can use nsIProcess instead.