As noted by others, Firefox saves partial downloads to .part
files. When the download is complete, the file is renamed. This is likely to prevent partial downloads from being mistaken for completed downloads. Other Mozilla-based browsers may use the same naming scheme. Chromium-based browsers appear to use a different extension (.crdownload
).
It is not normal to have .part
files left over after a download has completed. If you still have .part
files, the download is either still in progress or incomplete. Check about:downloads
in Firefox to see if they are still in progress.
In some cases, partial downloads can be resumed from within Firefox, but I have found it to be unreliable. External utilities are more reliable. For instance, wget
and aria2c
will resume incomplete files when the --continue
flag is used. The output file should be set to the partially downloaded file. When the download is complete, you will need to manually rename it to remove the .part
extension.
I have seen no evidence that any browser has ever used multiple .part
files for multi-part downloads. Although there appears to be a Firefox extension that allows multipart downloading, the description states "Make sure there is enough disk space. The files need to be first downloaded to Firefox's internal storage." This internal storage indicates that the extension does not use multiple .part
files.
External download utilities, such as aria2c
, often preallocate disk space and index download allocation separately (sometimes within another file, but with a different extension, not .part
).