let me suggest you something
- keep the same configuration: firefox and avast
- remove avast's web shield and install
avira browser safety for firefox (then, click on the icon -> settings -> disable all options) or
norton safe web (much lighter but a little bit weaker and more prone to false positives). I'm an avast fan but seriously, I hate avast's webshield because it consumes way too much CPU compared to the others. especially during download
- install
h264ify for firefox (an extension) which will significantly reduce CPU usage while watching youtube on youtube.com or other websites which use youtube