You're getting an HTTP error 203, which means the file that you're trying to download is zero bytes in size. Basically, you're trying to download nothing. It's probably an error on the part of the server, so try installing an extension from <a href="https://addons.mozilla.org/">here</a>.

Are you actually using Ubuntu, or any Linux distro? If you are, generally you can find the folders specific to applications in you home directory as cruft (hidden files that begin with "."). So on most distros, your Firefox preferences would be in ~/.mozilla-firefox; so to copy the file over, open a terminal and type
Code:
cd <i>directory-of-.js-file</i>; cp *.js ~/mozilla-firefox/<i>path-to-components-folder</i>
If you're using Windows, download the .xpi file (right click/save as, etc.) and save it to your computer. If that fails it's likely to be a server fault, and you could try looking for it elsewhere. In Firefox, open up Tools > Addons and drag the .xpi file into the extensions section. That should install it.
This method isn't reliable in Linux because it can depend on the location of your preferences directory and the permissions of the shell currently logged in.

As Odaisé Gaelach said, the guy in the link you provided downloaded the source code of Firefox, took the nsExtensionManager.js file and put it into the components folder of his current installation, which on Windows would be C:\Program Files\Mozilla Firefox\components, and on Linux will certainly vary, but check /opt/mozilla-firefox and ~/mozilla-firefox for the components folder anyway.