This is easily correctable.
- Install the 'Buttons!' add-on. You can search for it via Tools|Add-ons|Get Extensions (lower right), or simply go here: https://addons.mozilla.org/en-US/thunderbird/search?q=buttons!&cat=all
- Once installed, open an existing email message.
- Right-click any button on the toolbar and select 'Customize'. The 'Customize Toolbar' window will open.
- Drag the button that says 'Delete!', with the red X, from the 'Customize Toolbar' window to the location just to the right of the existing toolbar button that says 'Delete'. NOTE THAT THE NEW BUTTON HAS AN EXCLAMATION POINT; this is one of the new buttons you installed when you installed the 'Buttons!' add-on.
- Drag the 'Delete' button -- the one that DOES NOT have an exclamation point -- to the 'Customize Toolbar' window. Note you will likely have to move the toolbar window around, periodically, as you drag buttons to and fro.