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’s 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’s 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.