Me too. I assume it counts the major version number - which was indeed last updated a month ago - as opposed to 'minor' updates.

No, it's not doing that because I'm on Firefox Extended Support Release (which is currently at major version 10) and it says my browser is 33 days old.

Oh, then it's completely confused then.

Well, not necessarily because my browser might've got an update around that time. (Most updates to "Extended Support Release" will be security updates IIUC.)