[Therion] displaying models in a browser

Someone on UKcaving asked about displaying 3d models on the web, as opposed to making them available as downloads. Andrew immediately suggested generating vrml files and I see that Therion "appears" to be able to generate these. However, I cannot find a Firefox plugin that will display these, or X3D files. Has anyone else thought about this issue? It would be good to be able to display and manipulate models directly in a browser window. Graham