Have you tried delete button

It looks like this is a Pop mail account. It's generally impossible to delete a 'default' folder using the Delete folder option.

You would need to use the addon extension called 'folderFlags' which offers a new tab called 'Flags' in the 'Properties' of a folder. So the Junk folder has a Junk/Spam flag - it it has no glags then it's just an ordinary folder and can be deleted.

Otherwise, you can do this:

Exit Thunderbird - this is important Assuming you use Windows OS...and have Thunderbird profile in default location - note if Thunderbird was installed via App Store it will not be in default location.

• In windows search type : %Appdata%
• select '%Appdata% file folder
• click on 'Roaming' folder to see contents
• click on 'Thunderbird' folder to see contents
• click on 'Profiles' folder to see contents
• click on 'profile name' folder to see contents
• click on 'Mail' folder to see contents - assuming this is a pop account
• click on the name of the pop mail account folder

Look for 'Junk' mbox files - they are just files that do not have any extension. You need to keep the one with the biggest size Delete all the others and delete all the junk.msf files.

Then start Thunderbird and see if this impoves the situation.

Some images below as a guide to installing the addon. The last image shows right click on fodler and select 'Properties' In your case you wouldbe right clicking on Junk and unchecking all the flags. Then you should be able to delete the normal folder.