Using tor browser has nothing to do with Dark net. Good people use it to protect their privacy. You can use VPN as well.

Hacking your credential has nothing to do with TOR browser as well.
Hacking can happen if you are not aware of the security measures that one needs to know to use internet. There are phishing attempts, pop ups, unwanted malware visiting risky sites ans some many things. Anyone can inject a piece of program and can risk your security if you are not aware about minimum required knowledge when your are browsing online.

Are you saying you wan to use firefox with Tor connection?