If by overheating you mean the fans come on, and if you go into Activity Monitor you see a browser using a lot of CPU... What has been happening is that some websites have been using our computers for bitcoin mining... Other websites are simply poorly done and have processes or ads going crazy (nouvelobs.com never fails to do this, for instance). Security add ons like uBlock Origin and Disconnect have helped me get them under control with Firefox.
So it's not so much the macbook randomly overheating as specific websites playing havoc with computers in general. As has been mentioned, there are apps like istats that tell you the actual temperature.