I left the computer on overnight to download an .iso file. In the morning, there was still about 100MB of 600MB to download. I was doing some browsing when the internet stopped. I rebooted by router and it started again, but my download resumed from zero. Oh noes!
The partially downloaded .iso was in my Downloads folder with a .part tag, and Firefox had created a new, almost zero .iso file.
Fortunately, I found a way to get Firefox to resume the old, almost complete download here,although I had to adapt it slightly: pause the new download, delete the empty .iso file, rename the old .iso.part file to .iso and restart the download in Firefox.
It also seems to be possible to resume downloads with wget. This didn't work with the .iso.part file, but maybe it would have after removing the .part tag.