This is a scam attempted by some people who are targeting the most gullible of users (which is none of our readers are of course) into installing a backdoor Trojan that will log your passwords and possibly do unscrupulous things with them. Keep your passwords safe, and disregard this email should you receive it.
Users should always be aware that clicking links from unknown senders is not a good idea, and will generally lead to a negative result. Still it is worth warning about this one particular. Please remember that Firefox will update itself automatically, so if you ever receive an email or link telling you that your version of Firefox needs to be updated, please ignore it.
This email made the rounds this past weekend, and will probably continue to do so, so please be aware and delete it if you get one.
This is clearly one of the weaker attempts to get passwords, but there are people out there who will fall for such trickery. If you ever have a question about the validity of an email like this always refer to the official website. If you actually need to update to the latest version of Firefox do so here. (I promise, that is not a virus).
Source: Naked Security