The documentation for the webbrowser module at
http://www.python.org/doc/2.5/lib/module-webbrowser.html refers to the
use of the BROWSER environment variable as a means to control which
browser is run.  However, it makes no mention of how the browser name(s)
in this environment variable get translated into executables,
particularly the process by which the only directories to be searched
are those in the PATH environment variable.  

For instance, on my Windows PC where the default browser is IE, simply
setting BROWSER to 'firefox' has no effect.  I also need to add
'C:\Program Files\Mozilla Firefox' to PATH, after which Firefox actually
gets run.