The source explains that Firefox versions 28 and newer, which covers every modern release since around 2014, ship with the preference network.http.response.timeout set to 300 seconds by default. This default value appears to interfere with the ability to resume large downloads and can affect the overall stability of such transfers.
According to the source, changing the network.http.response.timeout preference to 0—or to a very large number—through the about:config interface seems to make big downloads both resumable and at least a little more stable. The author notes that they do not fully understand why this adjustment works, but experimental testing supports the observation.
The source emphasizes that this tweak only helps when the remote server already supports resumable downloads. If the server does not support range requests or similar mechanisms, no browser setting, including this one, will enable resumption.
Specifically, the problem the setting addresses is Firefox’s inability to resume a resumable download after an error occurs. The source observes that, without the adjusted timeout, Firefox may ignore an existing .part file that would normally allow a download to continue from where it left off. The author does not know the exact reason for this behavior but notes that experimental evidence points to a reliance on connection timeouts rather than the presence of .part files for determining resumability.
Resumable downloads in Firefox are described as generally unstable, a issue that has been known in some form since as far back as 2005. The source also mentions a manual workaround involving shuffling .part files to force Firefox to recognize and continue a download, but states a personal reluctance to use that method.
As an alternative outside of Firefox, the source reports that using the command‑line tool wget with the -c (continue) flag, combined with -O to specify the output filename, provides a stable way to resume large downloads: wget -c -O "$filename" "$url". The author finds wget to be generally reliable for this purpose.
Additionally, the source speculates that Firefox may perform hidden retry attempts before marking a download as failed, and that this retry behavior appears to be at least partly tied to the network.http.response.timeout setting. Exceeding the timeout could therefore interfere with those internal retries. The author has not verified these hypotheses by examining Firefox’s source code, citing uncertainty about where to look.
In summary, the source recommends adjusting the network.http.response.timeout preference in about:config as a practical step to improve the resumability and stability of large downloads in Firefox, conditioned on server support, while noting the broader instability of the feature and offering wget as a reliable non‑Firefox alternative.