I'm not aware of a way to join a popup window to the tabs bar of an existing window. There is a way to open a copy of the page (same URL), which would leave the popup open as well. Probably not so helpful, but you drag the lock icon from the left end of the address bar and when the mouse pointer is over the tabs bar, a blue place marker should appear showing where the new tab will be created. The about:config thing probably was a setting that forces '''all''' popups into tabs. Here's how you do that if you want to try it: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. (2) In the search box in the page, type or paste '''browser.link.open_newwindow.restriction''' and pause while the list is filtered (3) Double-click the preference to display an editing field, and change the value to '''0''' then press Enter or click the blue check mark button to save the change. If you change your mind, return to this preference and use the Reset button at the right end of the row (left-arching arrow icon). More info on about:config: [[Configuration Editor for Firefox]].