This may help. https://support.mozilla.org/en-US/kb/wrong-home-page-opens-when-i-start-firefox-how-fix

Hi Mary, based on your enormous screenshot in your other thread ([https://support.mozilla.org/questions/1417117 here]), the problem is that instead of the tab opening in the same window, it opens in a completely separate new window. I assume if you open the Window menu up on the MacOS menu bar, you see them listed individually there. How are you opening those tabs? For example: * clicking the "+" button on the tab bar * pressing the Command+T keyboard shortcut * holding down Command when clicking a link in a web page already open in Firefox * clicking a link in a separate program (such as Adobe Reader) that Firefox then opens in a new window * some other method Make sure not to hold down the Shift key when clicking a link or the "+" button. Shift will direct the new tab/link to a separate new window.