First, could you check to see whether Tracking Protection could be causing this issue? When Firefox's Tracking Protection feature is blocking content in a page, a "shield" icon appears in the address bar, to the left of the site's URL, next to the little i-in-a-circle icon. This article has more information about this feature and how to make exceptions: What happened to Tracking Protection?.

Before getting too far down the road, I wanted to mention a thread from earlier this year regarding a similar problem a user was having on the Google Pakistan site which unfortunately I was not able to properly solve: google.com more option (Google Apps) not working. In that case we checked:

• Firefox's Safe Mode
• Connection/proxy setting
• Whether scripts were blocked or not
• and more...

You might take a look at that one and see whether any of those steps are successful for you.

Thank you very much! This was the issue: the connection/proxy setting was set to "use the system proxy setting", now i've changed to "no proxy".

Now is working! Solved.

But... Why, after numerous installation of FF, the setting was always set to these values? It never happens in the past! (only now, that i've been turned to WIN10).

Thank you again! (sorry for the bad english)

On Windows, "Use system proxy setting" has always been the default. I'm not sure why the results are different on Windows 10. You can check your "LAN" proxy settings using IE or the Internet Options control panel if you are curious what is going on there.