Comment 14 for bug 1981163

Windows 10
Computer default open pdf using Adobe Acrobat

Thunderbird 91.3.0
Received email with pdf opens using Adobe Acrobat - expected so ok

TEST 1
Files & Attachments - pdf set to use Adobe Acrobat Dc default
Clear error console
Write compose window - add pdf attachment - double click to open -opens in Firefox in a new tab - not expected.
The url address shown in Firefox is:
file:///C:/Users/XXXX/Documents/ANCESTRY%20-%20Gibbons%20-%20Bell/The_Book_Gibbons_Family_Tree.pdf
Error console:
14:09:48.110
TypeError: requestor is null [Learn More] - PdfStreamConverter.jsm:140:13
getDOMWindow resource://pdf.js/PdfStreamConverter.jsm:140
onStopRequest resource://pdf.js/PdfStreamConverter.jsm:1262

Save as draft and close Write.

TEST 2
Change the Preferences
Files & Attachments - pdf set to 'Preview in Thunderbird'
Select Draft email - Edit
Write compose window - pdf attachment - same pdf as before - double click to open -opens in Firefox in a new tab - not expected.
The url address shown in Firefox is now:
file:///C:/Users/XXXX/AppData/Local/Temp/nsmail.pdf

error console repeats same error:
14:26:25.703 TypeError: requestor is null [Learn More]- PdfStreamConverter.jsm:140:13
getDOMWindow resource://pdf.js/PdfStreamConverter.jsm:140
onStopRequest resource://pdf.js/PdfStreamConverter.jsm:1262

Note: 'Learn More' is a link to this:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Unexpected_type

TEST 3
Tested with Write compose as Plain Text and also compose HTML - no difference in result nor error message.

TEST 4
Change the Preferences
Files & Attachments - pdf set to 'Always ask'
Select Draft email - Edit
Write compose window - pdf attachment - same pdf as before - double click to open --opens in Firefox in a new tab - not expected.
SAme result in error console.

TEST 5
Change the Preferences
Files & Attachments FIREFOX HTML Document - set 'Always Ask'
Clear Error console
Select Draft email - Edit
Write compose window - pdf attachment - same pdf as before - double click to open

Window opens to ask what to do - Open with - default is Firefox - so I change this to Adobe Acrobat and selected checkbox to always remember.
FRom now onwards Write compose window pdf attachment is opening in Adobe Acrobat
But this now means preferences are set up to open HTML in Adobe Acrobat which is not desirable.
Error console still producing same error.

We are getting reports in the Support forum of same issue or saying because the associated helper application does not exist.