Sounds like you're used to the way POP accounts work, but have set up the account as IMAP this time:

https://support.mozilla.org/kb/difference-between-imap-and-pop3

You may be able to set up the account as POP if you prefer (you'll have to set up the account again, cannot change the type of account once it has been set up), but I would recommend you to get used to the way IMAP works and learn to manage your mail the way you want using that protocol.

In particular, you may create folders under Local Folders, at the bottom of the list of accounts in the folder pane, to store and organise your mail locally on your computer there however you wish, independently of any account you may have.

Thank you. After reading your reply, I did look into the pop/imap issue, and this does seem to be my problem. I'm already upset with my new Thunderbird, because I also can't put a "delete" button at the top, where it works best for me, but instead have a garbage symbol beside each letter. This has caused me to lose a lot of letters too, because I accidentally end up deleting messages I didn't want to. I really want the pop method back, but am not sure that my ISP still offers that type of service. If it does, I'd much rather start anew with Thunderbird, than live with the IMAP system that really doesn't work for me at all. Thank you so much for your help!

OK, choose whatever works best for you, but FWIW you can put a 'Delete' button in the toolbar at the top if you want (View > Toolbars > Customize). I don't because I delete messages using the 'Backspace' key, don't see how a button would be preferable for that. And some of the other things you say don't even make sense to me…

I've tried to customize, but a "delete" button doesn't appear in the toolbar, just a garbage bin beside each email letter, to the right. Because I have to click on that garbage can icon to the right, a couple of times, when about to delete many of them, I kept clicking and went in the wrong direction, deleting letters I'd wanted to keep (and didn't realize it until much later. I get 100's of emails a day, and that delete button at the top made it very easy to just delete, delete, delete,delete, etc. I really appreciate your help.

Not sure what you mean. If you choose View > Toolbars > Customize, you don't see a Delete button available there that you may drag to the toolbar?

If I'm understanding it correctly, that bin icon you're seeing for every message is a column you may choose to display or not in Table View. Are you saying that you would prefer it to not be there or that you have intentionally chosen it to be there because you cannot figure how to have a Delete button in the toolbar?

And I don't really understand why that icon would cause you to delete messages you want to keep, but you don't have to use that or any other button to delete a message. Do you know you may delete messages using the 'Backspace' key without having to click anywhere?

When I choose view/toolbar/customize, I cannot drag anything anywhere. The full window is titled "customize toolbars" and has a variety of boxes below that can be "highlighted". I choose delete, and the "save" button is greyed out, until I then choose from a drop down menu called button style " Icons only, Icons above text and a couple of other things. I tried the Icons above text, and then the "save" button was hot, but nothing appeared on my toolbar.

Yes, I intentionally showed the garbage bin, since I couldn't create a delete button on the toolbar.

I don't understand what you mean about backspacing over a message? Everything I'm talking about is in the table view (if that is what my main inbox is called), not after I've opened each letter. I used to be able to delete all of these messages without opening them (just by seeing subject matter and sender), with a delete on that main screen. I'd love to learn more about this backspace method please.

When I choose view/toolbar/customize, I cannot drag anything anywhere.

You seem to be doing it wrong. You have to drag the button you want into the toolbar that has the search box, and then release the mouse either to the right or to the left of that search box to place the button there. Then, after having changed the toolbar somehow, the Save button will be enabled so you can save the changes. The Save button isn't enabled by merely selecting the Delete button because at that point you haven't changed anything yet, you've merely selected the button you want, but haven't placed it in the toolbar yet…

I don't understand what you mean about backspacing over a message?

Backspace is one of the keys available in any keyboard… It's the key, usually located above the Return key, used to delete text to the left of the cursor when you're typing… And then there is also the Delete key, used to delete text to the right of the cursor, that not all keyboards have, but that if your keyboard does have can be used to delete messages too…

You don't backspace "over" anything. You just hit the Backspace key on the keyboard when you want to delete the currently selected messages…

I can't believe I'm having to explain this and that you would get frustrated with Thunderbird for not knowing this…

This is what I get with a view/toolbars/customize: If I try to drag the delete to the top of this screen, it blocks me from releasing it, but I'm also sure that in the past, it wasn't the top of this page I had to dray it to. It allowed me to drag a delete to my main email page.

You have to drag it to the left or to the right of the search box that has "ctrl + K" in it, in the area occupied by "Flexible Space" represented by a dashed line with arrows at both ends. That whole bar is what you may customize, not anything else displayed in that window.