Put the command into a file with execute permissions, including whatever command-line arguments you want. For instance, the file might contain the following:

 #! /bin/bash
 xpdf -g 1200x900 -z width -cont "$1"

In Firefox preferences, under "Applications," locate the appropriate Content Type and in the Action menu, select "Use other..." and then locate the executable file you just created and select it.

In UNIX-type operating systems at least, available options to the command can be found using the "man" command (e.g. "man xpdf" for the example above).

How about for windows?

e.g. for PNG files I want to add the Helper App command line:

"rundll32.exe c:\windows\system32\shimgvw.dll,ImageView_Fullscreen "%1""

(which is the Microsoft RAW Image Viewer, same as its current windows File Association). Because the only options I'm offered on this machine now when I click on a PNG file link are to Save it, download it with the FlashGot plugin, or Open with the Browse button. On other windows machines here, Microsoft RAW Image Viewer is the default Open with choice.

Thanks.