Mobile is what matters nowadays.

This seems like a non-sequitur. Could you clarify?

We are forced to use only JavaScript features that are available on the system browsers of the mobile platforms, when doing web projects for mobile platforms.

So it does not matter what Firefox or Chrome support on the desktop, if those features are not widespread on the mobile platforms.

Are the mobile editions of Chrome and Firefox far behind in their Javascript support? I was under the impression that those two were pretty much up to speed with their desktop versions.

This does not work like that. It is the same thing as with IE 6.

You cannot assume everyone has a mobile device with the latest Firefox or Chrome version installed.

Most people only use the system browser, which can be whatever Webkit version depending on the iOS or Android version, Bink or mobile IE.

Some people might install the mobile versions of Firefox.

So as always, one is quite constrained if the goal is to target any mobile device with the same codebase.