Basic JavaScript quiz app
Design

You could remake this using Object-Oriented-Programming. For example, you could make a Question object, like this.

function Question(description, choices, correctChoiceIndex) {
    this.description = description;
    this.choices = choices;
    this.correctChoiceIndex = correctChoiceIndex;
}


You could then extend it with some methods, like askQuestion. For example, you could do something like this (Using prompt and alert because I'm on mobile.)

Question.prototype.askQuestion = function() {
    return prompt("Choices:\n" + this.choices.join("\n"));
}


You could then supply an additional method, checkAnswer, to see if the user got the answer correct.

Question.prototype.checkAnswer = function() {
    if(parseInt(this.askQuestion(), 10) === this.correctChoiceIndex) {
        alert("You got the question right!");
    }
    else {
        alert("Sorry, wrong answer.");
    }
}




Nitpicks

Some of your naming is not great. For example, firstFunc is an awful name for a function. It doesn't describe the purpose of the function in any way.

Preferably, names for variables/functions/classes should exhibit the following characteristics:


Describe the variable/function/class in detail.
Not have unnecessary abbreviations.
Not be too short.
Not be too long. (Unless you're programming in objective-c.)


Most of your variable names exhibit these characteristics, I just saw a few, like submitBtn that could be expanded to something like submitButton.

Moving on, I saw this line I your code:

var index = [i]; 


It isn't used anywhere in it's scope, and even if it was used, it would serve no purpose at this point.

Finally, when using the increment, ++, and decrement, -- operators, remember the differences between how they're placed.