The file is a driver from Nvidia, it's > 300MB so I can see that the whole file downloads before Firefox says 'failed' - the progress bar reaches 100%. IE has no problem downloading the file from the very same link - which then runs fine. This isn't the first time this has happened. Firefox creates the temp file and the empty file in the download folder as normal. Trying a 2nd time doesn't work - based on previous failed downloads. The problem seems to be some kind of file handle lock issue type thing. The file is 100% complete, I even managed to checksum the '.part' file and it was correct for the driver file. Win7 64bit Firefox 43.0.1 Download Manager No I will not reinstall Firefox, it is heavily customised and reinstalling it from scratch would take hours of tweaking. I'll just have to copy the .part file and rename it manually in the future until this is fixed.