Public:Email Server Info:Changing Thunderbird's PDF Preview
1. In Thunderbird, click on the 3 lines in the upper right and click Settings 2. In Settings, it should put you in General by default, but if not, click on General 3. Scroll down to the "Files & Attachments" section and search for PDF 4. In the box, on the right, under "Action" select "Open with [app] (Default)"
The app can be Acrobat, Chrome, Edge, Firefox, maybe even Irfanview - just not "preview in Thunderbird"