ComputerWorld reports that Firefox browser is not a part of Google Pack any more:

"Google Inc. today replaced Mozilla Corp.'s Firefox with its own Chrome as the default browser in the English version of Google Pack, the search company's application bundle for Windows.
...."

http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=9123345&intsrc=news_ts_head

Actually, it is not fully replaced. The content depends on language version.
Firefox is still there:
http://pack.google.com/intl/en/pack_installer.html?hl=en