Work around "Unresponsive script" dialog

This change sets two firefox config settings, dom.max_script_run_time and
dom.max_chrome_script_run_time, to 0, a value that will allow javascript
to run indefinitely without firefox invoking the "Warning: Unresponsive
script" dialog.  When this dialog appears, it stops selenium interaction
which, in the case of VDD, causes watchdog timer expiry.  Whether this
dialog will appear during the course of a VDD run seems to depend mostly
on the current server load, and, as such, is unpredictable.  At any rate,
this change hopefully will entirely eliminate this problem.

Testing done:

Checked about:config in firefox before and after the change to ensure the
values are getting set (it has proven impossible to reproduce the actual
problem locally, so that wasn't tested).