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SOLVED FF 19. What to Do? URL Link, ascm could not be opened, the associated helper application does not exist. Change the association in your preferences.


The error message above appears when I try to download an e-book file with the extension .acsm I have read many posts. At mozillaline site (? from 2008) http://kb.mozillazine.org/The_associated_helper_application_does_not_exist

Firefox 2: Tools -> Options -> Content / File Types -> Manage  (There are no File Types or Manage)

Firefox 3: Tools -> Options -> Applications (There is no ascm file or program to change, and no way to add to list) I tried http://kb.mozillazine.org/MimeTypes.rdf mimeTypes.rdf is a file in the profile folder. mimeTypes.rdf stores information about which action is to be performed when downloading certain types of files, such as opening the file in a specific program or saving it to disk. In Firefox, this information can be viewed or changed in "Tools -> Options -> Content / File Types -> Manage..." (There are no File Types or Manage and there is no method to Add)

The acsm extension is a common e-book file. I use Kobo Books.

Modified by popm

Additional System Details

Installed Plug-ins

  • Shockwave Flash 11.6 r602
  • Adobe PDF Plug-In For Firefox and Netscape 10.1.5
  • The QuickTime Plugin allows you to view a wide variety of multimedia content in Web pages. For more information, visit the QuickTime Web site.
  • npFFApi
  • Reader Library Helper Plugin
  • iTunes Detector Plug-in
  • ScorchPlugin
  • DRM Netscape Network Object
  • DRM Store Netscape Plugin
  • Npdsplay dll
  • np-mswmp


  • User Agent: Mozilla/5.0 (Windows NT 5.1; rv:19.0) Gecko/20100101 Firefox/19.0

More Information

  • Top 10 Contributor
2359 solutions 20892 answers

Helpful Reply

Usually if Firefox encounters an unrecognized type it will present Open/Save/Cancel dialog.

You might want to try renaming your mimeTypes.rdf file in case a plugin you removed is still listed in the file as a handler for that content type. Firefox will use a default file when it restarts.

Open your current Firefox settings (AKA Firefox profile) folder using

Help > Troubleshooting Information > "Show Folder" button

Switch back to Firefox and Exit

Pause while Firefox finishes its cleanup, then rename mimeTypes.rdf to something like mimeTypes.old

Restart Firefox. Does it behave better?

Question owner

Thank you for your reply. Your advice was spot on regarding the solution to my problem.

I followed your advice and was able to purchase a book and download it via the dialogue box. I saved it and moved it to my e-reader.

I have a related question. The size of the original mimeTypes.rdf is 18KB, whereas the new mimeTypes,rdf is 5 KB. Why is this so and should I be concerned?

  • Top 10 Contributor
2359 solutions 20892 answers

Helpful Reply

The mimeTypes.rdf file tends to accumulate entries over time. I think if Firefox doesn't find the answer in the smaller file, it will prompt you for information. I doubt that anything essential is missing. In theory, you could compare the files (for example, using WinMerge), but the format is not very self-explanatory, so I don't suggest trying to manually copy over any entries.

Question owner

Thanks for your information. I have marked this post as SOLVED.