Is it possible to assign a href/url attribute to a GraphStream Node such that when it's click, a new browser/tab opens to the specified URL?
Use ViewerListener to handle your graph events, then you can do what you want.
See here how it works : http://graphstream-project.org/doc/Tutorials/Graph-Visualisation/
Here you can see some examples with the last version : https://github.com/graphstream/gs-ui-javafx/tree/master/src-test/org/graphstream/ui/viewer_fx/test