I have a Fedora laptop which has run Thunderbird on KDE Plasma for years. I typically put the Thunderbird window on my big monitor where everything works as expected. When I move the window to the laptop screen (or unplug the laptop and use it without the big screen), I can navigate the folder list in the left-hand pane, but the list of emails and body panes on the right do not display, only the Welcome screen. When I put Thunderbird in Help > Troublehoot Mode, all works as expected. The only special add on I use is the Get All Mail button.