I am facing exactly same problem https://github.com/mozilla/geckodriver/issues/1358.
Firefox-headless mode screenshots taken from linux are missing input box
Question:
Do we have any alternative to solve this problem. I am using FF 83
This can be solved using firefox settings.
widget.disable-native-theme-for-content to true
Reference :
https://www.reddit.com/r/firefox/comments/k2nghh/any_alternatives_for_firefoxlinuxheadless/
PS: Credit goes to reddit user who answered my question :)
I verified it is working.