Recently a month back, Yahoo has launched new version for their Mail with new features such as conversations, disposable email address and others, if you’ve lost tabs that are useful for multi-tasking Yahoo is now offering "Recent" View for that. You can find Recent View below the Folders on left side of navigation.of inbox.
Recent View in Yahoo Mail
Recent view shows most recent activities:messages you’re composing, recently opened emails- only last ten recent messages will be shown- and your latest search. The keyboard shortcuts -Ctrl+[ and Ctrl+] on Windows and ⌘+[ and ⌘+] on Mac -which you’ve used in cycling between the tabs in the past still works for Recent View.
Yahoo is rolling out the feature, if you’ve not got it now soon you’ll.
If you want old Yahoo mail with Tabs do checkout this post.