I just installed Mountain Lion on an iMac and a Macbook. Both seem to be running smoothly... and I'm loving the update so far. Safari on the iMac has the iCloud Tabs button, and when I click on it I can see the tabs from the MacBook. However, the button is missing in Safari on the MacBook. I've checked that Safari is set to sync in the iCloud preferences... tried unchecking it and rechecking and restarting... but nothing seems to make the button appear. Safari is not set to "Private Browsing" either. Any ideas? Thanks for any help.