Javascript prompt() doesn't work in Firefox!

Javascript function prompt() (confirm() also) doesn’t work in Firefox (v. 70 /68 and previous vers. with ubuntu 16.04 or windows10) : after var u =prompt(“value ?”); the variable u remains undefined.
(try this example [](, you can modify the script for ex. erasing <if (person != null)> to see the undefined value)
On Chrome or Edge prompt() works as it must do.

This works fine for me in Firefox 70 (on Mac, though).

Do you see the problem in a clean profile?

Oh! Thanks a lot ! I (you) fixed the problem: I cleaned the profile and now all goes as it has to go. I’m curious to know which was the option that caused this problem, but now I have no time to compare the profiles.