Suddenly, ever since today, maybe yesterday, I don't recognize the display that I'm seeing when I open Thunderbird. It's some odd tab-like display (see attached example of what I see when I open TBird now), and I end up resorting to clicking the first tab, which usually has Sent or Inbox, something familiar. What am I seeing? I love TBird when everything looks familiar, but when suddenly I'm not seeing what I expect, I am just lost. Also, suddenly today, pretty sure today, not yesterday, when I double-click PDF attachments in emails I had sent, the PDF doesn't open, from what I can tell. When I then tried an MS Word attachment, that attachment opened fine. To try to solve this PDF-attachment-opening problem, I went into my Windows 10 PC's Control Panel > Programs > Default Programs > Associate a file type or protocol with a program > Choose default apps by file type ..... and then saw that the .pdf line item surprisingly said pdfs would open using "MICROSOFT EDGE" instead of what I would have expected to be either Adobe Acrobat or Adobe Acrobat Reader DC, since PDFs were opening fine in TBird some version of Acrobat before today. I then changed the .pdf line item from MICROSOFT EDGE to ADOBE ACROBAT. But, still, when I double-click a PDF attachment in a Sent email, nothing seems to happen. I then restarted my PC. And, in Thunderbird, I opened the Sent folder and right-clicked it, selected Properties, and clicked Repair. But, still, when I double-click a PDF attachment in a Sent email, nothing seems to happen. This probably isn't relevant, but . . . a week or two ago, I finally looked into the annoying behavior of PDFs ending up *downloading* instead of resulting in a dialog box that let me do a Save in Adobe Acrobat or Save in Adobe Acrobat Reader DC -- I forget how to exactly describe what was going on . ... but I had gone through some steps that actually succeeded in giving me the dialog box. Any insights on how to solve these two likely-not-related problems, step by step, I'd appreciate it!