Mail Front-End

A.K.A. the 3-pane tabs and message tabs/windows

In January 2023 the mail front-end was rebuilt from scratch, replacing what evolved from the original Netscape front-end. This is a developers' guide to the new UI.

Mail tab types

The mail front-end consists of two types of tabs (and a standalone window, more about that later) – the 3-pane tab mail3PaneTab and the message tab mailMessageTab. These are defined in mailTabs.js and provide the tabInfo objects for tabmail to control. Most code from outside the tabs will go through here in some form, although knowing the specific details should be unnecessary.

Each mail tab tabInfo object has these read-only properties:

  • chromeBrowser – This is a XUL <browser> object which displays the tab's contents, either about:3pane or about:message. As with any <browser> object you can access the displayed page with the contentWindow and contentDocument properties.

  • browser and linkedBrowser – Both refer to the XUL <browser> currently displaying content (an email message or a web page) to the user, or null if there isn't any.

  • message – The currently displayed message as an nsIMsgDBHdr, if there is one.

  • folder – The folder containing the currently displayed message, an nsIMsgFolder.

Accessing tabs

If the mail tab you're interested in is the current tab, the following properties of tabmail point to it:

  • currentTabInfo – The tabInfo object described above.

  • currentAbout3Pane – The window object of the page displayed in the chromeBrowser, if the current tab is a 3-pane tab.

  • currentAboutMessage – The window object of the message display page, which for message tabs is the page displayed in the chromeBrowser, and for 3-pane tabs a page within that.

If it's not the current tab, you can get the tabInfo object from tabmail and use the properties listed in the previous section to access it.

Mail windows

The standalone mail window also contains a XUL <browser> displaying about:message. The browser can be accessed from the window's messageBrowser property.


about:3pane is the main UI that users see when Thunderbird starts: the folder pane, the thread pane, and the message pane. It lives in the tree as about3Pane.xhtml and similarly named JS, CSS and Fluent files.

Folder pane

The folder pane displays the accounts and folders within them. Various modes of display are available.

Thread pane

The thread pane displays the list of messages in the current folder, and the Quick Filter bar for filtering those messages.

Message pane

This mesage pane contains more XUL <browser>s for displaying various things:

  • webBrowser – displays web content in a child process as Firefox does.

  • multiMessageBrowser – displays messages if more than one is selected.

  • messageBrowser – displays a single message using about:message.

Only one is visible at any given time.

Account central

If an account is selected in the folder pane instead of a folder, yet another <browser>, accountCentralBrowser – displays Account Central, a page of various things you can do in Thunderbird.


about:message is all of the UI that displays a single message, including the message headers and attachments. It is used as the message pane in about:3pane and by itself as a message tab or window. Like about:3pane it lives in the tree as aboutMessage.xhtml, .js, .css and .ftl files.

Message contents themselves are displayed in a <browser> (if you're counting, we're now three deep) which can be accessed by the content property of an about:message window.

Last updated