On Linux, the UX for retrieving attachments has been degrading as PDFs became more handled directly in Thunderbird. An easy step to ease the transition from Thunderbird to our OS's apps would be to add a pop-up notification to open the file. Example in Mattermost chat (you can't see the "open" tooltip on hover):
What's more, there is only one way to save a PDF that is displayed as a Thunderbird tab, through a click on the top-right save button:
Ctrl+S does not work. No "Save..." action in the right-click menu. This is a hastle for power usage!