Deprecated means that the object or method is put of date/old fashioned/or dangerous to implement and there is a good alternative available. Simply type the deprecation warning in google and you will get the things to get .
If you are using an external library then go to their website or forum and ask or search this
Offhand I believe the message you mention is technically a warning not an error. This should be gone in an update.
Even the newest versions of Firefox show it, even tho it shouldn't be shown as an error more as a warning.
You are getting this warning because you invoked a method belonging to the Components object. methods belonging to this object include classes, classesByID. See this for more explanation https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Language_Bindings/Components_object