If you still use Internet Explorer, there's more to worry about than a crappy browsing experience. Microsoft has confirmed that it's looking in to a "mouse tracking" flaw which affects IE 6 through 10.
The security problem was identified by a group called Spider.io earlier this week. It claims that versions 6 through 10 of IE are capable of tracking cursor movement even when the browser window is inactive, unfocused, or minimized—and that malicious ads could abuse that ability to track your mouse movements.
Indeed, Spider.io has shown that it's possible. It's not clear how useful the resulting data is—mouse clicks can't be recorded, for instance—but it should still be of concern to those worried about software security. Microsoft claims to be "actively working to adjust this behavior in IE". [Verge, Spider.io, Microsoft]