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.
Re: 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 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.