You can try disableing javascript, or installing the noscript plugin for
firefox (and disabling java there), or download the tor browser and use that
(also disable java)
Sometime mozilla will use 100 CPU wit java disabled for no reason seemingly.
Usually it's javascript tho. Browsing without javascript enabled is hundreds
of times more pleasant.
One way to disable unneccessary processes on Upstart using systems is to go
to /etc/init.d/ and make the script there un-executable.
I don't know how to use Upstart any other way.
SysV with rcconf is so much easier.
Upstart and Systemd both are trash, it's a shame everything uses it rather
than the SysV way.