[nitrogen]

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...

[lokedhs]

That's the one. Turns out that option is actually included in the main configuration menu: "Ctrl+Tab cycles through tabs in recently used order"

I have to say that the name doesn't explain what it actually does. Thank you for the assistance.