Which browser leaves which
Firefox writes .part next to the file being saved. Opera uses .opdownload. Some download managers also use .part.
Resume first
Open the browser's download list and look for a resume option. A resumed download is complete, while Crdownload can only work with what is there.
If resuming fails
Keep the partial file and do not let the browser clear it. Drop it into Crdownload, which works out the format from the content and ignores the extension.
Common questions
Can I rename a .part file?
You can, and a few simple formats will open. Videos and archives usually need their indexes rebuilt.
Why are there two files in Firefox?
Firefox may create an empty file with the final name beside the .part file while it downloads.
Sources
Documents consulted for this guide.