How do you open a message to read its contents? By double-click to open it in a new tab or a new window? Do you see the "Reply" button in the message header pane?

How to reply to a message?

Add a "Reply" button to the top toolbar and use it. Right click on the message and select "Reply" in the drop-down menu. Use the keyboard shortcut Command-r on a Macintosh. Control-r in Windows, I assume. Use the reply button in the message pane header.

Thanks! I right-clicked the message expecting a Reply or Reply-All

Many screen shots in Thunderbird forums are too small to explain or illustrate a problem. Yours is one of those since it shows a piece of a puzzle and not the bigger picture. It doesn't show that you have selected a collapsed message thread in the thread pane and then hit the R-click to open the context menu (see my image)

As far as I know you cannot reply to a message thread (or several selected messages) but only to a single message.

You might find a discussion on the forum about why using threads work better in table view than card view.

@johannes the context menu will be the same if you R-click on a collapsed thread in table view: there is no Reply option in the context menu (and there is no Reply button in the message header toolbar neither)