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Why your addon ask new permission?
The last is to communicate with software outside Thunderbird.

This addon is really good but don't ask too many permission.

Why you need to read AND modify the address book?

Это отзыв для предыдущей версии этого дополнения (2.7.0). 

Please check the FAQs:
http://mozext.achimonline.de/signatureswitch-me.php#faq

There also is a detailed changelog:
http://mozext.achimonline.de/signatureswitch-me.php#changelog

Plus, the source code of the add-on is also freely available:
https://github.com/4ch1m/mozext/tree/master/signatureswitch-me