Reason why it doesn't work on some messages Rated 5 out of 5 stars

I found one. If the headers section of the message is longer than 4096 bytes, then the extension doesn't work. This may have several reasons. For example there are many recipients of the message. Or some antivirus adds to much extra information. In my case it was the googlegroups.com mailing list which produced large headers. You can see the complete headers in message source code - press CTRL+U.

If the extension doesn't work for some message, just open Tools - Error console. You should see a message like:
hdr.get is not a function
If you can see it, then you probably have the same problem.

I have modified the extension so it accepts 32768 bytes which is a limit on many mail servers. It also contains two new reserved words TZ and TZ_NAME mentioned in my previous review. You can download it from http://rapidshare.com/#!download|47l32|442537343|smarttemplate-0.6.0-tb_timezones.xpi|36

I will contact the author so maybe the fix will be available in the next official version.

Timezones explained Rated 5 out of 5 stars

Great extension. Some explanation on time zones - local vs. sender.The following reserved words always use sender's time zone:
%date%, %datelocal%, %dateshort%, %date_tz%.

So if you want to use original time zone in your reply, then use them and you are lucky. But if you are like me, and you want to use your local time zone, then you must use other words. The following reserved words always use your local time zone:
%Y% %m% / %n% %d% / %e% %H% / %k% %I% / %l% %M% %S% %p% / %p(x)% %A% / %a% %B% / %b%.

As you however know, these reserved words return current date and time. But you can easily switch them to the time the original message was sent. Just place %X:=sent% before them. For example:
%X:=sent%%a% %b% %e% %Y% %k%:%M%:%S%
If you later want to use today's time, you can switch back by inserting %X:=today%.
For example:
%X:=sent%%k%:%M%:%S% vs. %X:=today%%k%:%M%:%S%
will produce two different times. The first time is the time when the mail was sent and the second one is current time. Both use your time zone and not the sender's one. Example output may look:
20:34:03 vs. 15:08:01

So it's almost everything we need. But I also wanted a timezone in the string, in the format:
Date: Thu Jan 13 2011 20:34:03 GMT+0100 (Central Europe Standard Time)

But there's no reserved word neither for your local time zone offset nor for your local time zone name. I could just manually put +0100 but it may change due to daylight saving. So I modified the version 0.6 and added two new reserved words:
TZ ... local time zone offset
TZ_NAME ... local time zone long name
So the desired pattern for the Date field with my local time zone looks as follows:
Date: %X:=sent%%a% %b% %e% %Y% %k%:%M%:%S% GMT%TZ% (%TZ_NAME%)

If you are interested in these two new reserved words, you can download modified extension from http://rapidshare.com/#!download|47l32|442537343|smarttemplate-0.6.0-tb_timezones.xpi|36