Really,Avastfan1,try to check your memory use with the default theme.
No-Script is a very fine add-on,but I really prefer to use "toggle Java Script",
which is one of the "buttons" of the add-on "toolbar Buttons"
I know that Java Scripts are not the only thing No-Script" protects against,
and I know you can get the same functionality from Firefox by going to tools+options+Content+disable/enable Java Script,but to me this is much easier,and it is Java Script that allows most of the fake anti-Virus scans to Hi-Jack a browser session.
In the long run,since no two people use a machine the same way,it is down
to trail and error to find what works best/lightest/most secure for you.
best wishes,
normishmael