So I take some time to do a more detailed analysis and these are my conclusions so far:
I created a VM with Win7 32-bit with IE10. I than installed FF version 38 followed by TS version 2014.3.1016 (the one I have license for :)). When and tried to execute a test in FF the same thing happened.
Afterwards I uninstalled everything and installed an older FF version (v34) followed by TS. Trying to execute again my test, I got some messages to install Telerik FF plugins and after I accept that I was able to run the test. Afterwards I upgrade FF to the latest version (v38) and I was able again to execute my test in FF.
Trying to do the same on my machine, uninstall FF and TS and install the same older FF version that in the VM (v34), followed by TS, I still couldn't execute the test.
Trying to find out if the problem was due to by machine, I created a new VM but this time with Win7 64-bit with IE10. I than installed FF version 34 followed by TS. I tried to run my test and this time I didn't get any message to install Telerik FF plugins and I got the same behavior (FF browser opens and just shows the message "Preparing browser for automation...").
This leads me to conclude that probably is problem is something to do with the fact I'm running Win7 c4-bit.
Did you ever did some troubleshooting regarding this? Or any client complain about this?