Try running Malwarebytes (Free) Version and Malwarebytes AdwCleaner as a second opinion scan.
You can obtain both applications here; Anti-Spyware/Malware & Hijacker Tools
You can also run the Zemana Anti-Malware tool as well.
Please let us know if this helps.
I tried these tools plus other 13 anti virus tools, still cannot solve this virus!
However, I used file explorer to find the path:
Deleted this xpi, then restarted Firefox.
It is ok now.
But why Firefox allows adding extension without my approval???
and Why many anti virus tools cannot fix this virus???
Those are questions above we volunteers You could ask in a firefox forum. Re why missed I will ask privately to someone we know if I get an answer i will update you.
I have asked in Mozilla Firefox forum, too.
I wonder why Firefox allows adding third party extension without the user approval/ consent/ notice!
This is a very danger loophole!