How to detect remote login

[Robert Voigt]

Can somebody think of a way to execute a command when a user logs in and
out remotely via XDMCP?

The use case is like this:
Several X sessions are running on PC A for different users. They have
Firefox and Thunderbird running. A user wants to log in via XDMCP from
PC B (X server) to their account on PC A. Firefox and Thunderbird are
started when the session is restored.

Now Firefox and Thunderbird can't have more than one instance running.
So I need to find a way to get a notification when a the remote session
starts, and then execute a kill command for the local Firefox and
Thunderbird instances.

Firefox and Thunderbird should be started locally again after the remote
session has ended.