While being deprecated and non-standard, the Google Gears version of offline Gmail was much better than this.
The primary reason this is Chrome-only is because other browsers don't yet have complete implementations of relevant specs (for example the FileSystem API: http://dev.w3.org/2009/dap/file-system/pub/FileSystem/). I can't make any promises on the team's behalf, but I personally would like to see it working on Firefox as well at some point.
They plan FileReader and FileWriter though.
Now I wonder: a) What is Moz leaving out? Why? b) Which of the other items from FileSystem does the Google team need, if any?