I'm on 68.0.1 and it hasn't done this forever. I haven't found a good enough reason to run older versions but I'm sure someone has.
With the newer Firefox versions, particularly 66, I kept having tabs suddenly becoming separate new browser pages. That I find extremely irritating. It seems now a webpage can "force" any tab to become a full new browser page if configured to do so, and Firefox default allows it. You can change it however. Found this posted just yesterday.
"I have always had this set, but suddenly links have started opening in a new window [another Firefox browser opens], and despite checking open in new tab is set, it continues. "
For future, putting the "'fix" here.
and put in the following at appropriate spots.
browser.link.open_newwindow , put 3
browser.link.open_newwindow.restriction put 0
browser.link.open_newwindow.override.external put -1