Background

There are a couple of problems existing in the way we do downloads right now that affect users in various ways:

it is not possible to resume downloads across sessions. If the user knows their network connection is going down they can use our limited "Pause" support now but only so long as the browser is open.
it is not possible to pause or resume downloads of components being installed via XPInstall, because the download progress notification back end is different.
there is much code duplication relating to file and path name validation in the browser and external helper app service due to the lack of a simple API for file transfer that coalesces all such code. This results in bloat and multiple sites to consider when addressing security issues.
performance of the download list is sluggish when the download history is long.
download list does not properly implement accessibility features that people are used to, does not scroll with key navigation, etc.

Features

The following features are required for 2.0:

cross session resume
storage back end
unified download back end with XPInstall, etc.
unified API for invoking download operations (e.g. unify code paths between link clicks and Save Target As... and other potential uses - single API to take a url and transfer)
accessibility - listview implements accessibility APIs and proper keyboard navigation support