A number of sites use Mac Keeper pop-under ads that:
1. Are atrocious and sometimes cause Firefox to hang
2. Annoy the fucking shit out of me
If you’re a Mac user, beware: never EVER install this software. It is straight from hell, the place where aliens and reptile lizards reside (curable only via magic drops of tap water in bottles for 12.95 each). Seriously, this is a very bad program and will fuck your computer to hell. It leaves little mouse turds everywhere that are almost impossible to get rid of.
If you’ve made the mistake of installing this crap, and your Mac is running slowly now, uh, guess what. Welcome to the gates of hell, aka Mac Keeper. Google “how to get rid of Mac Keeper” and find instructions to get rid of all the hidden pieces.