FireGPG is a Firefox extension which brings an interface to GPG functions. It's capable of signing, verifying, encrypting and decrypting text on the fly using GnuGPG.
It also integrates with Gmail, making it possible to run GPG functions on mail messages. One of its features is auto-verification of mail messages, which are detected as PGP-signed.
After a mail message has been verified, it is printed below the message whether the signature matches the message and if it does, issuer name is printed as well.
This type of XSS is interesting because a browser bug leads to an XSS. it is both similar and dissimilar to DOM-based XSS. It resembles to DOM-based XSS because the attack cannot be detected by the affected domain. But unlike DOM-based XSS, flaws of this variant are not originated from the developers of the affected domain.
Looking at FireGPG's case, Gmail is not responsible for the flaw, yet the XSS occurs under its DOM. Looking at Yair Amit's last post, the website which an HTML file is downloaded from is not responsible for the XSS, Internet Explorer is.
It should be noted that FireGPG's team has been very responsive and it took them about 21 hours since our disclosure to provide a remedy to the flaw. This shows they are security aware and are committed to the protection of their end-users. It is an excellent example of how to handle security related bugs correctly. Many vendors do not distinct security bugs from other bugs, IMHO this is a bad approach - security related bugs should be fixed as soon as possible, and not in the next service pack.