Firefox's 300-Second Timeout Breaks Resumable Downloads
Making big downloads resumable in Firefox
Firefox 28 and newer set network.http.response.timeout to 300 seconds, which interferes with resuming large downloads. Setting it to 0 in about:config makes downloads resumable and more stable, though only if the server supports resumable transfers. The author suspects Firefox relies on connection timeouts rather than .part files, and notes the issue has been known since 2005.
I don't understand why this works, but experimental testing supports it.