So you have mail that you want to save, but you don’t want it hanging around in your Inbox? Thunderbird allows you to create a folder that will store emails. Here’s how to do it:
Go to File > New > Folder
Give the folder a name and choose where you want to put the folder. To pick a location that already has folders in it (like your account name), keep going through the menus until you find “choose this for the parent”; that will choose that folder as the place to make your new folder.
Start moving messages to your new folder.