Rightclick your Email-Account and select 'Settings'
on the right side you can now edit your Siganture
Quick Folders
If you have a lot of folders, it's typically very time consuming to navigate through the tree on the left side, just in order to reach the folder.
With the Thunderbird Add On 'Quick Folders' you can configure your personal, most frequently used, folder list.
For example if you have to move your incoming mails into different folders, you just drag and drop the mails from the inbox to the configured folders in the 'Quick Folders'-List: