Download Plan is an interesting Firefox extension. It lets you download files at specific time intervals.
Here is more information about the add-on:
For some users, download files during certain hours is a headache either by, the speed of connection, limited bandwidth, peak times and schedule constraints on servers. For this reason it is appropriate to schedule downloads for specific times where these limitations diminish. With this addon, users can create a download plan to start at a desired time.
Want to learn more? Check it out at the Firefox Add-ons website.