Why are some tabs highlighted?

[inson1]

[navorite]

The distinct blue colour indicates the active tab on that window. So when a user saves a session, he knows exactly what tab had the focus in which window.

[inson1]

@navorite But active tab doesnt have to be the last, who has focus, right? for example There could be 3 active tabs in one window or more.

[navorite]

"Active" means the focused tab (the tab that is being currently viewed in that window.)

So for example, If I had 3 windows and one window had a Twitter tab on it which I'm currently viewing, that window (since it is the window being viewed currently) will be highlighted and the Twitter tab will be also highlighted to indicate that the user was viewing that tab of that window when he saved the session.

Now, for the other 2 windows that do not have the focus, they still have tabs that have focus, because each window must have a currently focused tab. Hope you get it.

From MDN:

active

boolean. Whether the tab is active in its window. This may be true even if the tab's window is not currently focused.

The active tab is usually the selected one. However, on Firefox for Android, extension popups open in a new tab. When this popup tab is selected, the active tab will instead be the one in which the popup opened.