User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.1 Safari/605.1.15

Steps to reproduce:

In a message with an attachment, click (or double-click?) on a .docx attachment to open it.

Actual results:

It produced the dialog shown in dialog.png, prompting me for what I wanted to open the file with.
The dialog was exactly as shown, with the "Do this automatically for files like this from now on" box already ticked.
Clicking OK opened the attachment in Word, as expected.

Expected results:

On clicking/double-clicking on the attachment, it should have automatically opened it in Word immediately, without prompting me.
Because I had previously ticked the "Do this automatically....." box.

We have Bug 1690395 for tracking the Thunderbird fallout.