A family of malicious Firefox addons have been discovered being pushed by sites pretending to be a manual update for Firefox. These sites utilize a combination of javascript alerts, user authentication prompts, and full screen views to try and coerce the user into installing the unwanted addons. Each domain has its own custom Firefox addon with names like Time Tracking, FF Helper Checker, FF AdBlock Protection, and FF Search Informer. When the addons are installed they will inject advertisements into web sites you visit, hijack links so they open advertisements instead of the desired location.
Source: https://www.bleepingcomputer.com/news/security/psa-beware-of-sites-pretending-to-be-manual-firefox-updates/