Years ago, this used to be the default in Firefox. Since Ctrl+Tab has not been the default, I've been using Ctrl+Tab extension. Other browser like Chrome do not provide APIs to allow extensions access to override tab order. Accessing tabs by order of last used, is a requirement for my daily web browsing, which is why I've stuck with Firefox for so long. I'm glad this is now part of the preferences, and does not require an extension.