For starters this does not happen very often but it is irritating never the less. Whenever I start firefox there is a strong hum and it seems that the CPU is trying to do a lot. Occasionally, the system also gives up and restarts.
Now, a restart does not sound good.
A top on the command line showed that gtk-gnash was hogging more than 80% of the CPU, not sure doing what.
Later I found this forum link too which seems to be about the same issue.
Anyway for now, I have uninstalled the mozilla plugin with
sudo apt-get remove –purge gnash gnash-common mozilla-plugin-gnash
and it seems to be working fine.