Download Scheduler is a free Firefox add-on to schedule Firefox downloads for a later time. You can specify a time when your downloads should happen, and then choose any download link to download during that scheduled time. This is perfect for those whose ISP limit the downloads during peak-time, but provide unlimited downloads during off-peak time. I know many ISPs in India offer such plans. In such case, you can schedule your downloads to happen during the off-peak window, so that you do not have to pay anything extra to your ISP.
Download scheduler plugin for Firefox is very easy to use. Once you have downloaded and installed this plugin, just specify the download window in plugin options.
After that, whenever you want to schedule a download, just righ-click on the download link, and select option of “Schedule Link as”. This will add that download link to scheduler. At your scheduled time, the download will start. You can schedule as many links as you want. Of course, if you want to download something immediately, just follow the normal download procedure of clicking on a download link. Also check out how to schedule downloads in Chrome.
To download during scheduled time, make sure your Firefox is open during that time. This plugin works with Firefox 4 as well.