Do I understand well that there is NO WAY to use the ONLINE version with Chrome or Edge ?

Yes, Edge never supported Java plugin and Chrome stopped supporting it (Java plugin is a NPAPI plugin).

Running windows 10, noway to get IE

As far as I understood, Edge is the default browser under Windows 10. That doesn't mean that IE doesn't work under that system. I just tried again Sweet Home 3D Online with IE 11 and it worked without problem.

So I would have to install Mozilla ?

Firefox will remove support for plugins too at the end of 2016, so until then it should work.