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Mozilla Foundation Security Advisory 2006-26
Title: Mail Multiple Information Disclosure
Date: April 13, 2006
Fixed in: Thunderbird 18.104.22.168
As a privacy measure to prevent senders (primarily spammers) from tracking when e-mail is read Thunderbird does not load remote content referenced from an HTML mail message until a user tells it to do so. This normally includes the content of frames and CSS files, but CrashFr showed it was possible to bypass this restriction through indirection: the direct CSS or iframe src is included in-line, with that including remote content.
Switch the "view message body as..." choice to "Simple HTML" or "Plain text". Or for POP mail, after downloading the mail switch to offline mode before reading any of it.