Comment #8 on issue 1023 by lasiewski: Firefox 4 Support http://code.google.com/p/gears/issues/detail?id=1023
The blog entry mentioned by @v5point0 is 8 months old. The Google Apps still do not support HTML5 Web Storage[1], and there has been very little communication from Google regarding their plans for offline applications.
Since Google Gears has been abandoned, and Web Storage for Google Apps is not available for Firefox 4 (and other browsers?), this means that offline mode for Google Mail, Google Calendar and Google Docs is currently dead.
[1] http://en.wikipedia.org/wiki/Web_Storage
