I just got the same thing. I checked the properties of the email and it was sent from a unusual named server that doesn't look like one I would expect from Mozilla/Firefox. In fact when I tried to track down the "Inmail,sk" (note: calmma was used instead of a period to prevent a click-able link), I ended up at an infected web page. Fortunitly my anti-virus caught and blocked it and if it had not I'm using the Firefox add on No-Script, which would not allowed any scripts to run.

My advice would be Don't visit any Inmail,sk sites and to block emails from r3,rs,email-od,com ([173.203.166.158]). (Period was also replaced in the previous string.) Also I doubt the Mozilla people can track this back to the source with the little info we have provided here.