block3.js

// Assume that this code is executed inside the mutation observer callback:
addedNodes.forEach(node => {
    // See above for script tag checks…

    node.type = 'javascript/blocked'

    // Firefox has this additional event which prevents scripts from beeing executed
    const beforeScriptExecuteListener = function (event) {
        // Prevent only marked scripts from executing
        if(node.getAttribute('type') === 'javascript/blocked')
            event.preventDefault()
        node.removeEventListener('beforescriptexecute', beforeScriptExecuteListener)
    }
    node.addEventListener('beforescriptexecute', beforeScriptExecuteListener)
})