Provides a button and keyboard shortcut (Shift + Alt + Del by default) to move selected messages to the Trash folder regardless of the mail account's When I delete a message setting. It was developed to improve the experience of using GMail via IMAP.
The recommended option for that setting is Just mark it as deleted, which preserves the concept of Thunderbird folders acting as GMail labels. Deleting from a folder should only remove that label, and leave the message archived under All mail. Moving a message from any folder to Trash (or Junk) will result in permanent deletion (after the grace period), also removing all other labels.
Regular Shift + Del is unhelpful with GMail because it works similarly to normal delete, only removing a label without the desired permanent removal.
It is not recommended to use Move it to this folder: Trash as the general delete setting because it breaks the folders-as-labels convention. With that, the only way to remove a label would be to move a message to the All mail folder, where it already exists, which is very unintuitive.
You can edit the default shortcut under Add-ons and Themes > Extensions > ⚙️ > Manage Extension Shortcuts > GTrash.