(please excuse my poor English, I'll try to make myself clear as much as possible, thanks!)
I'm not an advanced user. I want to block any internal javascript (Internal Style Sheet) code, but allow all external .js files
You can disable inline javascript using the UserCSP addon in Firefox. If you set up a CSP for a site it will by default block inline JS and functions that create js from strings like eval.
https://addons.mozilla.org/en-US/firefox/addon/newusercspdesign/