Select the same tool, whose icon has slightly changed and whose tooltip now says Disconnect from Browser.

If you had originally opened the browser through one of the Engineering Links tools or by choosing Connect Web Browser, selecting Disconnect from Browser closes the browser in addition to disconnecting it from OpenUtilities Substation . If you had opened the browser from outside OpenUtilities Substation and then connected to it, selecting Disconnect from Browser only disconnects OpenUtilities Substation from the browser without closing the browser.