Firefox has native .watch() method.

The watch() method watches for a property to be assigned a value and runs a function when that occurs.

Interestingly, it’s special to Firefox.

Warning: Generally you should avoid using watch() and unwatch() when possible. These two methods are implemented only in Gecko, and they’re intended primarily for debugging use.

I discovered this when I was using watch for an option property name options.watch = true. My stuff worked fine in Chrome & Safari, but it was absolutely broken in Firefox.