Firefox Portable Nightly has a corrupted or an incomplete download file. when downloading straight from portableapps.com, a 1.6MB installer is downloaded and while using that, it shows ''ERROR. file not found 404''. using portableapps.com platform to install nightly shows the error message. only in the case of ''FIREFOX PORTABLE NIGHTLY'', other Firefox variants are fine.
There was an issue with the original build. It's been fixed with a rebuild and reupload. Apologies for the inconvenience.
Sometimes, the impossible can become possible, if you're awesome!