Press the button, wait for an alert to pop up, then press it again.

Version FFr2

When the button above is pressed, a request is sent to developers.mozilla.com. Onprogress (or onerror), a function r1() is called, which increments the variable i by 1, then alerts its value.

Apparently, in Firefox Focus on Android it happens that if the name of the variable is i, FFF resets it to 0 each time. By renaming it to something else, for example test, its value increases each time.