I honestly have no idea why you're seeing that message, Tanath. This did not pop up in all the tests the QA company did when testing Hidden Folks on Linux. I can't find anything regarding Unity & Firejail on google, don't know what Firejail is, and I don't have a Linux computer or any knowledge of Linux to be of any help. If you can figure out what Unity wants from Firejail, I think that's your best bet in solving this issue.