Test Error: Invalid browser - Firefox on ami-373b804f

[thedoc31]

I've set up a WPT Private Instance with auto-launching agents. Chrome works fine, but if I try to launch a test with Firefox, I get a result of:

The testing completed but failed.
Invalid browser - Firefox
First View: Test Error: Invalid browser - Firefox

My config section in ec2_locations.ini looks like this:
[us-west-2]
browser=Chrome,Firefox
label="US West (Oregon)"
region=us-west-2
ami=ami-373b804f
size=c4.large
min=1
max=10
subnetId=<customsubnet>
securityGroup=<securitygrouplist>

Any ideas?

[thedoc31]

I realized perhaps I need to be more specific. This is a problem on the NEW wptagent instance, not the older (deprecated) WPTDriver instances with IE10,9, etc.