Change javascript code in FireFox using Inspect Element

Here is javascript code :

<script type="text/javascript">
    var javascript_countdown = function () 
    {
        var time_left = 10; //number of seconds for countdown
        var keep_counting = 1;
        var no_time_left_message = 'Please Activate Javascript.';
        function countdown() 
        {
            if(time_left < 2) 
            {
                keep_counting = 0;
            }
            time_left = time_left - 1;
        }
        function add_leading_zero( n ) 
        {
            if(n.toString().length < 2) 
            {
                return '0' + n;
            } 
            else 
            {
                return n;
            }
        }
        function format_output() 
        {
            var hours, minutes, seconds;
            seconds = time_left % 60;
            minutes = Math.floor(time_left / 60) % 60;
            hours = Math.floor(time_left / 3600);
            seconds = add_leading_zero( seconds );
            minutes = add_leading_zero( minutes );
            hours = add_leading_zero( hours );
            return  minutes + ':' + seconds;
        }
        function show_time_left() 
        {
            document.getElementById('javascript_countdown_time').innerHTML = format_output();//time_left;
            document.getElementById('javascript_countdown_time_bottom').innerHTML = format_output();//time_left;
        }
        function no_time_left() 
        {
            //document.getElementById('javascript_countdown_time').innerHTML = no_time_left_message;
            //alert("");
            document.FormAzmoon.submit();
        }
        return {
            count: function () {
                countdown();
                show_time_left();
            },
            timer: function () {
                javascript_countdown.count();
                if(keep_counting) {
                    setTimeout("javascript_countdown.timer();", 1000);
                } else {
                    no_time_left();
                }
            },
            init: function (n) {
                time_left = n;
                javascript_countdown.timer();
            }
        };
    }();
    javascript_countdown.init(601);// Time Recorder Secends
</script>

This javascrip code is a reverse timer(10 minutes) in a page.
When timer = 0 it submits the page auto.
I want manipulate it for increase time.
I think best way to increase time is this line :

setTimeout("javascript_countdown.timer();", 1000);

change to :

setTimeout("javascript_countdown.timer();", 100000);

When i change this value on Inspector of Inspect Element in firefox nothing happens.
So what is the solution for that?
If there is another way other than firefox please teach me!
Also teach me how can i rewrite and manipulate a variable inside a function via console!

JS will get parsed and run as soon as it's in the DOM. Changing the page markup after the JS has run will not change the running of the JS. You'll need to change it some other way.

One option would be to overwrite the .timer function with your own function. After the page script has run, open your console and run:

// save a reference to the original function if you need to
const origTimer = javascript_countdown.timer;
javascript_countdown.timer = () => null;

This will prevent the page's calls to .timer from doing anything. Then, once you want to submit the form, just call document.FormAzmoon.submit(); manually.

Also teach me how can i rewrite and manipulate a variable inside a function via console!

If you actually had to resort to that approach, it's just barely possible, see this question for details. In short, you'd need to use a userscript (or something else that runs at the beginning of pageload), use a MutationObserver to intercept the addition of the <script> tag, and fiddle with its textContent once you detect the addition of the script.