A few months ago, Yahoo refreshed its mail service, which meant adding some features and removing others. Tabs were cut after feedback from users suggested that they were difficult to deal with in large numbers. Many were unhappy when they were removed entirely; you can't please everyone it seems.
Tabs are now back and allow users to see them all at once in a tiled view, thanks to the "Preview All" function. This makes it easier to view tabs and switch between them quickly. There's also a drop-down menu available next to each tab, allowing users to switch to another tab that way, close a particular message, or close them all instantly.
The new functions will be rolled out to users in the coming days. We've gotten our hands on the Tabs feature, and it's a welcomed return!