there are several yellow folders

What does this mean? Please post a screenshot illustrating the error?
https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Did you make any changes causing this?

I keep getting new folders added. One would think that selecting the folder icons, then clicking 'delete' in the dropdown menu would indeed delete them. That does not work!!! What is the "Archives" icon at the top???

It looks like somehow you have managed to get a copy of a 'pop mail account folder' into a 'pop mail account' folder. In the past I've only seen that occur when someone tried to copy a pop mail account folder into a new profile and managed to put it in the wrong place - inside an already existing pop mail account.

The repeat occurances in the 'Trash' folder look like you have repeatedly tried to delete the 'pop.mail.yahoo.com' folder.

In Thunderbird

• Help > Troubleshooting Information
• click on 'Open Folder' button

A new window opens showing the contents of your 'profile name' folder.

• Exit Thunderbird now - this is important.
• Click on 'Mail' folder

It will show you a list of your pop mail accounts and also 'Local Folders' mail account folder. One of those pop mail accounts will be called 'pop.mail.yahoo.com'

• click on 'pop.mail.yahoo.com' folder to see contents.

You will see:

• Inbox (no extension) mbox file
• Inbox.msf
• Drafts
• Drafts.msf
• Sent
• Sent.msf
• Archives
• Archives.msf
• Trash
• Trash.msf

All of the above are good files that are required, do not delete any of them.

You may also see 'pop.mail.yahoo.com' mbox file and a .msf file and also .sbd folder of same name and also a 'Trash.sbd folder.

If unsure on what to do next please post an image showing what you see.

• Delete the 'Trash.sbd' folder.
• Delete the 'pop.mail.yahoo.com.msf' file
• Delete the 'pop.mail.yahoo.com' (no extension) mbox file

You may see a 'folder' called 'pop.mail.yahoo.com' or 'pop.mail.yahoo.com.sbd'

• Create a folder on your desktop call it 'TB Stuff'
• Move the folder that mention 'pop.mail.yahoo.com' and put into 'TB Stuff' as temp backup.

So now the only files you see are those mbox and .msf mentioned above as good files.

Start Thunderbird.