Resumable downloads in firefox generally appear to be fairly unstable though. This has been a known problem in some form since as far back as 2005.
It's odd how the experience differs so much; I clearly remember successfully resuming a download less than a month ago, and I don't remember any time Firefox had any problems with them. My network.http.response.timeout is the default of 300.
There are reasons, but until someone actually publishes an article that's more detailed than the vagueness here, there's nothing anyone can really do about it.