A family of malicious Firefox addons have been discovered being pushed by sites pretending to be a manual update for Firefox.
Since learning about one of these addons from Pieter Arntz, I have found over 100 domains being utilized for this scheme. 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, and inject Monero miners into Firefox. As you can see these are not very pleasant addons. To make matters worse, they do not allow you to access the Firefox addons page, so it makes it harder to remove them.
A small sample of the domains pushing these addons include:
For a demonstration of this scam and how it injects code into sites, you can view the video below.