The source code is available at https://github.com/akhodakivskiy/VimFx.
The source code is licensed as follows:

All ".coffee" files are licensed with GPL-3.0+. That is also explicitly noted at
the top of each file. A copy of the license is included in the file "COPYING".

All ".tmpl", ".xul" and ".json" files are in the public domain. So are all ".md"
files, except the ones in the "documentation/" directory.

All files in the "documentation/" directory are licensed with GFDL-1.3+.
That is also explicitly noted at the top of each file. A copy of the license is
included in the file "documentation/GNU Free Documentation License".

All files in the "extension/locale/" directory and its subdirectories are in the
public domain. Translators are credited in "PEOPLE.md" and the "About" dialog of
the extension.

All images are copyrighted by Heidi Xu 2013. They may be distributed as part of
this extension.

VimFx has a few dependencies, listed in "package.json". They can be downloaded
into the "node_modules/" directory by running "npm install". Each of them have
their own license and instructions on where to get their source code.

Instructions on how to build VimFx from source are available in the
"documentation/tools.md" file.