text entry - very strange behaviour in firefox

Hi everyone, I've uploaded a course to my Articulate Online account, for testing purposes. In the course is present a classical text-entry box for the username, that I put over an image to create a "fill the form" effect.

The testing on my PC goes all right, with Firefox 44.02. On the client "side", however, with the exact same version of Firefox, the text-entry won't work as expected.

The text-entry box shows the placeholder text, then when the user clicks in, he can not enter his name. The flashing vertical bar for text-enter does not show. Strangely enough, if the user minimizes the window and then reopens it, everything works fine. I suggested to update the Flash version, but this did not solve the problem. Testing with Chrome and IE is just fine. Has anyone encountered a similar, odd problem? Any advice on how to solve it? Or I simply have to tell the client to not use Firefox? (suggesting a "minimize and then reopen" troubleshooting does not sound very professional to me).

Everything works fine in local, too.

I searched the forum but did not find mention of this very peculiar problem.

Thanks,

Dario