Some F-key assignments don't "Bind"

Using Tab-Mix (like tons of people do) and I've noticed that periodically (every half hour or so) the F-key assignments for "close tabs to the left", "close tabs to the right" have to be reset. They don't actually "bind," in other words... The F key assignments I use are NOT in use for any other Firefox menu commands at all... I tried assigning them from within Tab-Mix, no go...