I am no MAC expert but, you can use firefox in mac too, with all the plugins for adblock and noscript. Use it when you visit suspect pages, and maybe the exchanges too.

Steam should be safe if you install it from the legit page.

Also you could have no rootkit, and only you pressed on the wrong link when entering myetherwallet, if you reach the wrong web then they can copy your private key, they are always telling this:
Always check the URL and look for MYETHERWALLET LLC [US] Certificate up there. Look for https://www.myetherwallet.com/.
Check your web history if you went to a different page but that was alike.