Online DWF viewer Autodesk Design Review QuickShare (now included in Autodesk 360 cloud services) works only in the newest versions of web browsers supporting HTML5 and WebGL.
The Firefox browser supports these standards since version 4.0 (better in 5.x). Make sure you have the current version of Firefox. Plus your graphics card (its driver) must support WebGL and first of all the WebGL support must be enabled in Firefox.
This setting can be done on the Firefox configuration page: in the address line type about:config and scroll down in the parameter list (listed alphabetically). Verify or change (by double-clicking) the settings of the parameter webgl.disabled - should be se to false (OK in most cases), and set the parameter webgl.force-enabled to true (this is the source of 3D problems in most cases).
You don't need to save anything, just switch back to the page with CAD data and refresh it (F5). Now the 3D mode should work.
You can verify your settings on the bicycle 3D model: