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.