If you're reading this, it's likely that you want to run a local instance of the configuration converter. This is encouraged and advisable; while I can personally guarantee that we don't keep what you upload (nor would we need or want to) on our online instance, we released this tool under an open source license so that you can run it locally and modify it as you see fit. We even include a commandline tool you can use so you don't even need to host the website interface! (Which is a good thing; uwsgi is not for the faint of heart.)
The quickest way to get started is to make sure Python 2.6+ is installed at the bare minimum. The web interface (app/{views, models, etc.}.py) and commandline interface (app/fpconvert-cli.py) have both been tested with Python 3, and both work just fine.
It's highly recommended that you install the LXML module as well; it offers much safer processing. Since you're only processing one configuration file and it's (presumably) your own, though, this isn't as much of a concern; it'll work just fine with the Python standard library modules.
To use the tool from the commandline, simply execute the app/fpconvert-cli.py application - it only takes one (required) argument, the path to your foxyproxy.xml file. See our online instance for more info on where to find that file; it should be in the base of your Firefox profile directory. It has been tested and works in GNU/Linux, macOS, and Windows 10.
Note that if you wish to use Python 3, you must execute it as follows (according to what the binary is called on your system):
python3 fpconvert-cli.py path/to/foxyproxy.xml
The web interface is written in Flask. You'll need to have the module installed for the version of Python you want to use. To run a local instance, simply run:
FLASK_APP=run.py flask run
in the root directory of the project. If you want something more persistent, however, you'll need to use something like uwsgi. Unfortunately, that's a major undertaking that's out-of-scope for this documentation. The web interface, both Flask execution and uwsgi deployment, has only been tested in GNU/Linux.