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

All ".coffee" files are licensed with MIT. A copy of the license is included in
the file "LICENSE-MIT".

All ".md", ".tmpl", ".xul" and ".json" files are in the public domain.

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.