There is a thread at WebmasterWorld where an AdSense publisher posted a screen shot of the AdSense ads on his site being formatted incorrectly. He wrote, "I have about half a dozen ads showing across my various sites that appear to be broken."
He posted some screen shots and it looks like the CSS and formatting is broken on these ads:
Google has been recently testing new AdSense ad formats, so maybe there are some issues with some of them on some sites?
Forum discussion at WebmasterWorld.
Here is a sad update:
@rustybrick Finally heard back from G on this - it's an AdWords compatibility bug with Firefox, and no ETA on a fix. Nice.— netmeg (@netmeg) May 17, 2016