bit of an update to this.
looking at activity monitor the fans are coming on when there high cpu usage, but the usage varies by browser. A good example is the homepage of Plex, www.plexapp.com. Nothing fancy about this page but it will cause the cpu to max and thus the fans:
FireFox - 100% cpu and max fans
Safari - 60% cpu - quietest
Chrome - 15% cpu although there in another process called Google Helper that runs at 95% - max fans
In fact, the same page wil cause high cpu load on my iMac, but the fans won't come on.