mmmh, weird approach

You have the solution

but it is not properly highlighted

Q: How to run Sweet Home 3D online on Firefox?

R: If you have Java Runtime installed, just click this

http://www.sweethome3d.com/SweetHome3D.jnlp

Though I'm wondering why this is not SHARPLY given as alternative when clicking the Online button on the navigation bar of this website [:-/]