if you hold shift while clicking to start Thunderbird to get it into safe mode. Do you still have the issue?

Mark, This did not solve my problem. I've also come down with not being able to access my key board on the right. (Numbers lock not on). Do you think there is a correlation here? Both happened about the same time. Marie

You do not have to use double click to open emails.

A single left click will open the email in the Message Pane usually displayed below the list of emails. This is not a preview - it is an opened email, so if you can single left click then this is an easier method of viewing emails.

A single right click offers you other options that you select with single left click eg: Open in a new tab or window.

As you can do both of these, it seems to point to your 'double-click' action which the computer cannot understand or is not receiving, so cannot act upon it.

I think this issue sounds like a function not working with either your mouse or Touchpad and not an issue with Thunderbird. First check your mouse or touchpad settings via Control Panel to see if the settings need resetting or adjusting. If you have a spare mouse or can borrow one for testing purposes then test to see if the double click works.

I have tried everything all have suggested. No luck. I decided to do a system repair to a previous date. Presto. Solved both of my problems. I don't know why or how but who cares. I works now.