From MozillaZine Knowledge Base

"Advanced Folder Views" were added in Thunderbird 2.0. Thats a folder pane header that lets you select a predefined folder view such as favorite folders or recently viewed folders. You can remove it by adding the following to the optional userChrome.css file in the chrome directory in your profile.

/* Remove Advanced Folder Views */
#folderPaneHeader {
display:none !important;
}