If you need to install Canvas 3D, you'll need Firefox 3.5 or later. Go to https://addons.mozilla.org and type 'Canvas 3D' into the search box. If the extension is still considered 'experimental' (it was at time of writing) then check the "Let me install this experimental add-on" box. Then click the "Add to Firefox" button - which will download and install Canvas 3D.
When you first visit a site that uses Canvas 3D, you may see a window banner that says "A web page at XXX would like to create a Canvas 3D context" - you'll need to click the "Allow" button.
There is a test program here: http://people.mozilla.org/~vladimir/canvas3d/examples/modelview/ModelView.html
- Details are at: http://www.c3dl.org/