Thomas: I'm experiencing the same thing in Safari (I'm on a new Mac mini running Sierra). Every time I open a new browser tab, it opens to search.yahoo.com instead of my favorites/frequently visited sites. Also, when I click on a link in an email from a trusted sender, it will open to the same page and not follow the link.
My wife wanted to figure out what was going on and wound up at [link removed], which seems sort of scammy to me.
I activated Malwarebytes on this Mac tonight, quarantined two files, and have been declared "clean," but still get redirected to yahoo.
Thanks!