Add SELENIUM_FIREFOX_PATH for sad-path environments

I have Firefox Nightly earlier in my path than Firefox stable. Nightly
has problems with Selenium that haven't been fixed, yet, (or something
like that). This allows me to specify exactly which Firefox binary to
use without having to fiddle with my PATH environment variable which
caused other problems because of virtual environments and such.

It defaults to exactly what happened before, so if it worked for you
before, you should see no change with this change.