See https://support.mozilla.org/en-US/questions/1221659

Thanks for the redirect. I had already been through that thread and the suggested solution. It may have worked for previous releases, but it does not seem to work for me in TB78.

I tried using a path like C:\users\userid\desktop\file%20name.pdf (should there be a space) and it plugged the URL for the document into the attachments box, but when I saved, closed, and then reopened the event - no attachment (not even the link to the attachment).

I also tried using a path like fil://c:\users\userid\file%20name.pdf

Same result - no attachment saved (it again showed it initially - as evidenced by the image, but when saved, closed, and reopened, nothing).

Any additional thoughts?

I think you have to use the format given here:

http://forums.mozillazine.org/viewtopic.php?p=14774450#p14774450

such as file:///C:/Users/John/Documents/Alpha%20Beta/test.pdf

I added a jpg attachment this way to an event and saved the event. When the event was opened and the attachment double-clicked, I was prompted to specify the opening app, and then the app opened, but not the jpg itself. As the Bugzilla report indicates, this feature is not complete.