User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/93.0.4577.82 Safari/537.36

Steps to reproduce:

1. Set the file type of pdf to open with adobe pdf reader in Thunderbird
2. Read a message with a pdf attachment. Double-click the attachment and it opens in pdf reader. (this is expected operation)
3. Now forward the message that has a pdf attachment
4. Before sending the forwarded message, attempt to open the pdf attachment by double-clicking it, or right-click open. A browser will open to a blank page at link "file:///C:/Users/<user-name>/AppData/Local/Temp/nsmail-2.pdf" and the file will not open in the pdf reader.

Actual results:

Before sending the forwarded message, attempt to open the pdf attachment by double-clicking it, or right-click open. A browser will open to a blank page at link "file:///C:/Users/<user-name>/AppData/Local/Temp/nsmail-2.pdf" and the file will not open in the pdf reader.

Expected results:

The file should have opened in the pdf reader.

Changing the pdf action to "Preview in Thunderbird" and then back to "Use Adobe Acrobat Reader DC (default)" resolved the problem.

Correction, changing the pdf action to "Preview in Thunderbird" and then back to "Use Adobe Acrobat Reader DC (default)" did NOT resolve the problem. The problem happens when attempting to open a PDF from the message compose window when forwarding a message with a PDF attachment. The problem does not occur if opening the PDF file when reading the original message.

For using the internal viewer - bug 1734428.