javascript in the firefox browser

I am supporting a web form .net application and I have found out that some of the JavaScript has been deprecated using Internet explorer by using the Internet explorer web developer tools. I have now used the Firefox browser and the application seems to work for what I have tested so far.

Before I place the new version of the application into production, I would like to know what JavaScript features are currently obsolete and/or may be deprecated in the near future.

Due to that fact, would you tell me and/or point me to urls (links) that will tell me what JavaScript items are obsolete and/or deprecated using the Firefox browser?