From MozillaZine Knowledge Base

Some misunderstandings

I removed a couple of misunderstandings.

Dtobias wrote:

Since recipients might not be using the same operating system, in the same configuration, that you do, it is not safe (or conformant with standards) to use any characters outside the US-ASCII range, which basically limits you to unaccented letters, numbers, and standard punctuation (but no "curly quotes" or em-dashes, since these are not in US-ASCII).

That is not how signatures work. Thunderbird reads the signature file into the editor. The editor works in Unicode, so it supports all characters. When Thunderbird sends the message, it encodes the entire message for transmission, including anything that might have originally come from the signature file. Therefore using any characters at all in your signature file is safe and complies with standards.

Dscotese wrote:

Thunderbird allows you to set a signature file under Tools | Account Settings for each of your identities (called "accounts" in Thunderbird).

In Thunderbird, identities and accounts are not the same. A signature file is an identity setting, not an account setting. Unfortunately, Thunderbird's user interface does not make this clear. For more information, see: Mail concepts