Firefox 28 and newer versions have network.http.response.timeout set to 300 seconds by default, which interferes with resuming large downloads and affects download stability. Setting this timeout to 0 or a very high value in about:config enables resumable downloads and improves stability, based on experimental testing. This fix only works if the server supports resumable downloads; otherwise, no browser setting can help.
The issue appears to stem from Firefox relying on connection timeouts rather than .part files for resumable downloads, causing it to ignore existing .part files without the setting change. Resumable downloads in Firefox have been unstable since at least 2005. A manual .part file shuffle can force resumption, but the author prefers not to use it.
As an alternative, using wget -c -O "$filename" "$url" provides a stable workaround for resuming downloads. Firefox may perform hidden retries before failing, and exceeding the timeout likely disrupts this process, though the author has not verified this in the source code.
Source: Hacker News · Summarized by HeadlinesBriefing