...invece di usare l'apposita estensione di firefox.
Si può?

Cioè (io uso linux/ubuntu) apro il terminale è digito

java -jar selenium-server-standalone-3.4.0.jar -htmlSuite "*firefox" "" "/mytestsuite/mytestsuite.html" "/mytestsuite/results.html"

ma mi esce questo errore

Unable to find the HTML runner. This is normally because you have not downloaded
or made available the 'selenium-leg-rc' jar on the CLASSPATH. Your test will
not be run.
Download the Selenium HTML Runner and
use that in place of the selenium-server-standalone.jar for the simplest way of
running your HTML suite.