Comment 25 for bug 1981163

Reports occuring in Support Forum.
https://support.mozilla.org/en-US/questions/1359046

Either pdf is auto opening in firefox
OR getting error message "Attachment could not be opened, because the associated helper application does not exist. "

Due to pdf's being seen as HTML documents.

Some workarounds exist:
see comment #8 and comment #9

If getting the error message set up the 'HTML' document to 'Always ask' or choose the actual pdf program.

Hopefully the developers are working on discovering why the pdfs are seen as HTML and why it is neccessary to alter the Content Type: HTML document to use Adobe or Ask.