if you are using a wireless mouse, try changing the battery in the first instance. They do odd things as they get to the point of failure.

This is a frustrating issue, but it is a known behavior in Thunderbird—especially after recent updates—that usually relates to how the message list is indexed or how input is being processed. Since you've already checked your mouse settings, the issue likely lies within the application's database or a specific view setting. Here is a professional troubleshooting guide to resolve this: 1. Rebuild the Folder Index (Most Likely Fix) If the index file (the database that tells Thunderbird which message is which) is corrupted, clicking "Delete" on one email might accidentally trigger the action on adjacent ones because the "map" of the folder is misaligned. Right-click on the affected folder (e.g., Inbox). Select Properties. In the "General Information" tab, click the Repair Folder button. Thunderbird will rebuild the index. This won't delete your emails; it just refreshes the list. 2. Disable "Threaded" View Thunderbird sometimes defaults to "Threaded" view after updates. If messages are grouped in a thread, deleting the "parent" message or a collapsed group can unintentionally delete the entire conversation. Go to the View menu (or the hamburger menu ≡). Select Sort by. Ensure Unthreaded is selected. 3. Check for "Message Grouping" If your messages are grouped by "Date" or "Priority," the selection behavior can sometimes "stick" to adjacent items. Go to View > Sort by. Uncheck Grouped by Sort. 4. Troubleshoot Add-ons (Safe Mode) A specific extension might be interfering with your click actions. Go to Help > Troubleshoot Mode... Click Restart. Test if the erratic deletion continues. If it stops, one of your extensions is likely the culprit. 5. Hardware Latency (Double-Click Issue) Even if your mouse settings seem fine, a "micro-bounce" in the mouse switch can register a single click as a double or triple click. Test: Try using the "Delete" key on your keyboard to remove a message instead of clicking the "Trash" icon for a few days. If the problem disappears, the issue is likely the hardware switch in your mouse or trackpad. Pro Tip: If this happens again, you can immediately press Ctrl + Z (Windows) or Cmd + Z (Mac) to "Undo" the deletion and bring the messages back instantly without going to the Trash folder.