Does that also happen in Troubleshoot mode?

Ronald?

Hello there

Did you missed updates? What was your last one?

Do you use the latest version?

https://www.thunderbird.net/en-US/desktop/

Hello there

1.check layout settings.

maximise your windows settings first Click the menu button/3 striped hamburger. Go to customise>layout> Make sure the folder pane is selected(checked). Alternative , use the shortcut f8 to toggle the folder pane /view on/off. Reopen thunderbird after that.

2.resize the pane.

If it’s visibly but tiny move your mouse to far left of thunderbird. When the cursor changes to a double headed arrow , click and drag to the right to expand the view / pane.

3.temporary disable the add ons that might interfering the program by deleting them , using trouble shoot mode is good program to use it for that.

Tips :

1.Try to restore the folder pane :

in that left hand column itself is missing, view>Layout and check folder pane. It s possible to toggle this with f9. Restart thunderbird ,reopen thunderbird. Reopen thunderbird after that.

2. Switch back to the Inbox Tab:

Look at the top of your Thunderbird window. When you select an email, it may open in a new tab, making the inbox "disappear." Click the Inbox tab to see your folder list again.