Sounds like appropriate behavior, in that the fans should increase in RPM as the internal temps increase. Firefox is known to cause CPU usage to increase (happens on my PMG5 consistently).
istat pro is a wonderful dashboard widget for troubleshooting this issue - http://www.islayer.com/apps/istatpro/
Also, run Activity Monitor, with open processes sorted by CPU usage. Between these two apps, you should be able to identify the process hogging the CPUs.
Are you using the machine on a hard surface (like a desk), or on a soft surface (lap, bed)? If you are using the machine on a soft surface, you might be inadvertently blocking the intake ports on the bottom of the machine, as well as preventing the bottom case from acting as an effective heat sink.