doc/format-patch: adjust Thunderbird MUA hint to new add-on

There are three tips how to compose a non-line-wrapped patch with
Thunderbird. The first one suggests use of an add-on. The one
referenced has long been superseded by a different one. Update the
link to the new one. Mention that additional configuration is
required to make the add-on work.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: j6t

Documentation/git-format-patch.adoc

@@ -587,13 +587,19 @@ an external editor to keep Thunderbird from mangling the patches.
 Approach #1 (add-on)
 ^^^^^^^^^^^^^^^^^^^^
 
-Install the Toggle Word Wrap add-on that is available from
-https://addons.mozilla.org/thunderbird/addon/toggle-word-wrap/
-It adds a menu entry "Enable Word Wrap" in the composer's "Options" menu
+Install the Toggle Line Wrap add-on that is available from
+https://addons.thunderbird.net/thunderbird/addon/toggle-line-wrap
+It adds a button "Line Wrap" to the composer's toolbar
 that you can tick off. Now you can compose the message as you otherwise do
 (cut + paste, 'git format-patch' | 'git imap-send', etc), but you have to
 insert line breaks manually in any text that you type.
 
+As a bonus feature, the add-on can detect patch text in the composer
+and warns when line wrapping has not yet been turned off.
+
+The add-on requires a few tweaks of the advanced configuration
+(about:config). These are listed on the download page.
+
 Approach #2 (configuration)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Three steps: