Want the old tab behavior back? There’s a simple about:config fix.
Open a new tab (:P) and type
about:config into the address bar. You should end up in the configuration key editor. Paste
browser.tabs.insertRelatedAfterCurrent into the “Filter” field. This should narrow down the listing to a single item. Simply double-click the row to toggle the setting from “true” to “false.” You can then close the tab and go back to your browsing.