@lxo @nemobis @fijxu >thanks to librejs and noscript, I could check it first.
Unfortunately, even with those you could not check it first due to the way the JavaScript engine is designed.

There is no guarantee that the extensions will run before the JavaScript start running and JavaScript structures that will bypass then keep being found.

The only way to be safe against site JavaScript in firefox is to go to about:config and set javascript.enabled=false.

>but it would still be running under someone else's control
Browsers could give you the opportunity to check all JavaScript before it is executed, save a version you prefer and use only that version, or run a different program with that website instead if you want, but such functionality has not been implemented and has been made needlessly difficult to implement intentionally.