Hi,
It is but natural that when you download some suspicious downloads, be it firefox or any browser, when you click and nothing seems to be really downloading, these kind of Potentially wanted adds on crept to your add on. It may be surprise, that you did not get alert from useful firefox add on alert. It bypasses all the security and copies the add on as always active one.
Normally this will not harm any computer programs but reside in as if nothing is happening, but it may be sending your personal data for viewing thro unknown members, who are there without having any useful work.
I would advise you to scan your pc thro adware cleaner , effective tool to get rid of these kind of programs. It will be surprising that even after adware cleans and delete the harmful entries, some of the folders still reside in the system.
This is normally an affair with the crack programs free websites. The exe will instead of downloading the programs it advertise, install PUP and those nonsense browser hijacks. Pl clean your pc with adware cleaner as i do not think the malware bytes at present form did anything special.adware cleaner immediately catch hold of those.