Currently we are unable to set geckodriver for Firefox. I see on latest version of Firefox, starting from version 80 next issue when try to use browser.switchToWindow() method (Java, JS - result is the same):
Expected "handle" to be a string, got [object Undefined] undefined
Currently I see version 0.24.0 on Saucelabs. This issue does not appear on 0.27.0 version. Please update 0.24.0 to 0.27.0.