It's likely just a rogue ad, a screenshot would help tremendously in tracking it down.
Oh, and Firefox sucks. Especially as it relates to malware and phishing issues.
I'll get a screenshot the next time it happens.
Firefox has worked perfect for me for years and I'm running malware/virus software.
I used to run FFox exclusively but have had a lot of crashes so went back to chrome. Btw not getting any voice ads, site is at times balky