Hi,
Did you install your FF not in default destination (C:\Program Files\Mozilla Firefox)? I was also in that situation but after adding firefox.exe to Path, Katalon did start FF without error. To check if FF is added in your Path correctly you can just launch a command line and type “firefox” + ENTER there. If firefox starts then everything is alright and you can try to restart Katalon or your computer.
However, if you’re using FF Portable, I don’t think it works. Try to re-install official FF here.
Hope it can solve your issue.