If you mean cards view, no.

Switch to Table View by clicking the icon next to the Quick Filter button (see picture). If the button is hidden, enable Message List Header under View/Layout.

I'm just starting with Thunderbird. Is there any way to display each inbox message on one line instead of two?

Use Table View rather than Card View. It shows one line per message and it's much more flexible. You can choose what information is displayed and reorder the columns as you like.

I don't understand the point of the new Card View. Each entry takes twice the space and provides less information and no flexibility. Perhaps it's a development path towards a smartphone version of Thunderbird.

Our family has been using Thunderbird for quite a while now. However, the recent update to 115 isn't helpful (or are there still bugs with basic functionality now, months later?) For example, the really annoying display of the first line of an email in the list of emails should be only opt-in - currently it seems the user can't even opt-out (in spite of claims it should be possible). In today's (5 Jan 2024) new install of Thunderbird (115.6.0) on a newly installed OS (Linux Mint 21.2, 64-bit) there is 1) NO Table View anywhere in the menus 2) NO icon next to the Quick Filter button 3) NO card view in a drop-down which can be de-selected. In short no method to get rid of that annoying first line (it's wasteful too). The only solution appears to be to go back to the previous version (or choose another email client). We do admire and are very thankful to those fantastic people who write Thunderbird, but it is super annoying when things which have worked so well for ages are suddenly changed and there is no clear, intuitive way to opt-out of really annoying changes.