This is a list of extensions I find useful to have installed in Thunderbird:

• British English Dictionary
• Enigmail - Handles PGP signing and encryption of mail. Requires gnupgp or similar to be installed on the system. Doesn't like anything but text/plain format emails though.
• Lightning - Outlook style calendaring for Thunderbird.
• Mail Redirect - Allows you to bounce (rather than forward) email to another address, amongst other things.
• Provider for Google Calendar - Bidirectional sync to your Google calendars. Doesn't always play nicely; need to spend some time working out why.
• QuoteCollapse - Uses folding to hide away the quoted parts of emails so you can see the real content. Useful if your contacts insist on replying below the a quoted message (which happens to be the Thunderbird default too).

Thunderbird has a similar advanced configuration editor to the one found in Firefox. It can be accessed via:

Tools→Options→Advanced→General→Config Editor

The advanced config editor will be needed to apply most of the hacks documented on this page.

By default, Thunderbird sorts IMAP folders by date ascending, so that new emails appear at the bottom of the mail list. For reference, this is the date sent, not date received; mail sent from a machine with a broken system clock will not appear at the correct place in the folder.

To invert the default sort order, open the config editor and use the filter to search for the mailnews.default_sort_order key. Change the value to 2, for descending sort order.

The field on which the folder is sorted can also be changed using the mailnews.default_sort_type key.

The following keys are related to this hack:

mailnews.default_sort_order mailnews.default_sort_type mailnews.default_view_flags mailnews.default_news_sort_order mailnews.default_news_sort_type mailnews.default_news_view_flags

The following values are acceptable for the above keys:

sort_order:

byNone        17         byPriority    23      byLocation    29 
byDate        18 *       byStatus      24       byTags        30 
bySubject     19 *       bySize        25       byJunkStatus  31 
byAuthor      20 *       byFlagged     26      byAttachments 32 
byId          21 **      byUnread      27      byAccount     33 
byThread      22         byRecipient   28      byCustom      34 
  * = commonly desired values
 ** = by Order Received (?)

sort_type:

ascending   1
descending  2

view_flags – the second group of values can be added to one of the first group to combine effects, with several limitations:

Unthreaded       0
Threaded         1
Grouped         64  [[mail|only (?)]]
ShowIgnored      8  [[news|only]]
ShowUnreadOnly  16
ShowExpanded    32  [[doesn't|seem to work]]

This hack was brought to you from https://bugzilla.mozilla.org/show_bug.cgi?id=86845#c74.