Firefox - no "open with" dialog

[Steven Heimann]

I have a number of computers now running edgy.  On only one of them when
you click on a link in firefox that points to a file like a pdf you get
a dialog only asking where to save the file.  There is no option to
"open with" an application.

I have been unable to locate an option in about:config to control this
behaviour.  I can not understand why this computer would behave
differently to all the others.

Thanks and regards
Steven