It is under Preferences (to get it click your username or the itch logo(not in browser in border), or go to your itch icon on your taskbar(Windows and Linux only (I think) and right click it) then scroll down and check Enable tabs. It's that simple!
It might be on the Mac! Haven't used a Mac since 2017 so idk.
And also I just used the itch app for the first time yesterday, so I don't know.
I'm on Windows.