Just downloaded 7.0.1.6/installed/saw this same 6000 error, after a request for my registration code. Also had the software appear to crash 2-3 times ("encountered a problem and needs to close...")
I uninstalled/redownloaded/reinstalled and again had an immediate 6000 error, again accompanied by a request for my registration number--but...it's freeware, right?!
No perceptible "run" occurred; this was, on all occasions, immediate.
Running XPpro, FF2.0.0.6
--pants