The solution in this post is not mine; I found this solution by stumbling across a Selenium Grid discussion on Google groups (the thread can be found in my references below). Initially I thought that the issue existed with my Selenium Grid/OS (firewall/SELinux) setup as Firefox was launching but not processing tests.
This entry is here to help direct people to the correct answer.
Selenium Grid agents running Firefox do not appear to be working. A single Firefox window may launch but then it just hangs, or a single window may launch and then close and no additional Firefox windows are opened.
On the Selenium Grid agent, on the terminal you just see heartbeat pings (i.e. no updates on running tests)
INFO - Ping Hub at http://localhost:4444/heartbeat?host=localhost&port=5555
The issue exists between Selenium Grid 1.x (and more specifically the older version of Selenium RC/Server that it ships with) and newer versions of Firefox (versions newer than 3.6 — i.e. 4, 5, and 6 at the time of writing) . It appears that Selenium RC/Server 2.0 is required to support newer Firefox versions.
The solution involves upgrading the Selenium RC/Server shipped with Selenium Grid to the newest version (i.e. 2.7 at the time of writing) — I found that steps 1-3 were sufficient for me.