The Firefly repository is
Caltech-IPAC/firefly on GitHub
API documentation is linked from the first page README.
The standalone firefly with embedded tomcat server is in
Releases
Assuming the tomcat server is running locally on port 8080,
http://localhost:8080/firefly - brings the front page
http://localhost:8080/firefly/demo/ffapi-2images.html - basic example demonstrating how to work with images, show the regions, and track the mouse via API
http://localhost:8080/firefly/demo/ffapi-highlevel-test.html - much less tidy but more complete showcase of current API functions
NOTE: Please, note the change fftools->firefly