In Firefox, if you open the download window while the file is still downloading, the download percentage is shown on screen. It may be that this changing status wasn't announced previously but is now, I can't recall, although I thought it was. In any case, this is not related to status bar progress announcements.

Regardless the solution is simple: don't watch the download in progress. In other words: if you start a large download, go browse somewhere else and don't open the downloads window (which Firefox calls a library).

Luke