How to fill forms in firefox

[curb]

How do I fill forms in firefox?

Here's an example url - https://html5doctor.com/demos/forms/forms-example.html

Code:

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int w=wait(3 WV win("Forms Complete Example - Mozilla Firefox" "MozillaWindowClass"))
Acc a.Find(w "TEXT" "First Name First name only" "state=0x14100000 0x20000040" 0x3015 3)
a.SetValue("asd")

Error received:
Error (RT) in fill form:  0x80004001, Not implemented.