One thing to check is Task Manager (ctrl-alt-delete) open it, then try something in Firefox and switch to Task Manager 'processes' and look to see exactly what is using CPU... it 'may' be that your firewall or AV is set to scan a bunch of stuff. If it IS Firefox itself, then you need to go thru your menus to see what you have set.
   I am just trying the 'noscript' add-on, which allows setting controls on individual pages...with many options... but it requires some twiddling and study. Plain old AdBlock might be enough.
It's hard to diagnose without sitting there, clicking options...