These "roll over ads" are not restricted to Firefox and Safari, they happen on all the browsers.
Ok, I'm getting really annoyed by those weirdly placed blue words/phrases on web pages I cruise, the ones that when you roll over them, a pop-up appears with something related to the word, but usually utterly irrelevant to the page you are viewing. Is there any way to kill the phenomenon? I can really do without an ad for Toyota when the cursor passes over the acronym "SUV" or pop up about some TV show when the cursor passes over somebody's name.
The phenomenon seems to afflict both Safari and Foxfire.