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 OpenBuildings Designer. If you had opened the browser from outside OpenBuildings Designer and then connected to it, selecting Disconnect from Browser only disconnects OpenBuildings Designer from the browser without closing the browser.