Unless something has changed recently deleting Firefox also deletes anything you ever downloaded with it. Be very careful about this.

Also there is no way to clear the list of downloaded files, there is only a 'delete' button, which if clicked deletes all your downloads. This is particularly annoying if you start downloading something only to find out you already downloaded it (i.e a podcast), the download history tab is the only place to cancel the download, but if you've used firefox for a while the tab is so slow to load that your download has often completed before the tab has loaded.