Installing untrusted addons into a browser is a bad idea. It's like installing malware directly. They can potentially escape from the browser through exploits that are way easier with more rights.
In this case they could do just about anything.
They could commit fraud in your name by stealing from websites you visit through YOUR computer, YOU could be prosecuted for hacking.
Or they could steal from you by changing your information (shipping adress) or steal your credit card/banking data directly.
additonal source and info: https://security.stackexchange.com/a/98278/77793