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.
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.