Checking Firefox Version

I am trying to write a script in Javascript* which would inform a visitor's using Firefox if they are running an older version. Although Firefox does automatically check if there is an update and informs the user, I have seen visitors with versions like 0.8, 1.0.x, 1.5.y (y<11),>

Automatic updates are helpful for home users at least as corporate users might not have the rights to upgrade their software without administrator access. However, informing them with a visual warning on a web page could help in avoiding potential problems of using vulnerable versions.

Presently, the script checks the CVS branch tag i.e. rv: value with a hard coded value and prints a warning for users with older versions. Is there any way that the hard coded value(s) can be acquired automatically from say mozilla.org server and checked with visitor's user agent information? I don't know much about AJAX but would it be helpful in this case? No AJAX compatibility issues as the only browser we are checking is Firefox.

* - using Javascript because it runs client side, is available to all Firefox users and those who have disabled Javascript would most likely not need the warning to know it.