Firefox has a Object.prototype.watch method

Introduced by: https://github.com/Leaflet/Leaflet/commit/d70d03dbe1772e039eae2ba8785926f1fa8ada57

Firefox ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/watch

Thanks :)

(And hi from NDjamena, where I'm running an openstreetmap/Leaflet workshop :) )