If you want to keep your Mozilla Thunderbird Inbox and other folders sorted by what you’ve read and what you haven’t read, you may sometimes want to mark them all as read. There’s a quick way to do this.
Mark all emails in your Inbox as read.
To quickly mark all messages in a Mozilla Thunderbird folder as read:
Select the folder or a message within the folder.
You can also choose to select all emails, right-click and choose Mark > Read.