I think it's about:config -> browser.ctrlTab.recentlyUsedOrder set to false. If that doesn't work, try searching the web for "firefox disable new tab switcher" or similar.
I also saw a browser.engagement.ctrlTab.has-used key in there; didn't know the config store was also tracking usage...
I also saw a browser.engagement.ctrlTab.has-used key in there; didn't know the config store was also tracking usage...