The associated helper application does not exist
From MozillaZine Knowledge Base
This article deals with causes of the message "(filename) could not be opened, because the associated helper application does not exist. Change the association in your preferences." Firefox stores file type and application associations separately than Internet Explorer does, which can explain why one would work and not the other.[1]
Application uninstalled
This error message can mean that the program that Firefox thinks should open a specific type of file is no longer installed. You can reconfigure Firefox to use a different program:
- Firefox 2: Tools -> Options -> Content -> File Types -> Manage
- Firefox 1.5: Tools -> Options -> Downloads -> View and Edit Actions
Find the entry relating to the file type you're trying to open, then click Choose Action... and pick the application you want to handle that type.