Out of curiosity, I Googled it even though I don't use Firefox. Mozilla Support has a thread on it started almost a year ago. They say it's malware and not from Firefox; so, don't run it or save it. They haven't posted a definitive solution. Apparently it's a complex problem because the malware originator adapts it. You can read Mozilla's recommendations here:
https://support.mozilla.org/t5/Probl...ate/ta-p/37696
You can read the entire thread on the subject and contribute to it if you wish here:
https://support.mozilla.org/t5/Firef...30940#M1014901