The latest update to macOS Sierra, released on January 23, 2017, has a bug which may remove a very useful feature: iCloud Tabs. This feature allows you to view tabs open in a browser on another one of your devices, Mac or iOS. These pages sync via your iCloud account, and you can check a web page, say, on your iPhone, then later, when you get back to your desk, see it there. (As long as you didn’t close it on the iPhone.)
You can view iCloud tabs from the cloud button in the toolbar:
You see a list of pages, by device, and you choose one to open it. These tabs are also not visible in Exposé view in Safari in the desktop as they were before.
To restore iCloud tabs, go to System Preferences > iCloud, uncheck Safari, then check Safari again. The iCloud Tabs button will return, and you’ll be able to access your iCloud tabs from that button, or from Exposé view, and you’ll see tabs and pages open on the desktop on your iOS devices.