I'm with DarknessFalls on this one and i strongly suspect that strict mode didn't work for you, because you are not wrapping your code in an anonymous function. Strict mode is enabled or disabled per function and since the base scripts don't use it, your plugin is inheriting their "strict mode disabled" state.
The fact that it works in some browsers is probably implementation specific, since you aren't intended to use strict mode outside a function, anyway. So, if you follow the best practice, wrapping your code in an anonymous function and placing "use strict"; as the (important) first statement inside that function, it will work.