Html – autocomplete user/password in firefox

I've created a login submit form in HTML but for some reason user/password autocompletion does not work like I expect in firefox.

This is what happens in Firefox:

• I give username and password and click on the login button
• Firefox prompts me if I would like to remember the password. I press 'remember' and login works. (I made sure I deleted all remembered passwords before running this test)
• I log out and return to the login page. I would expect the username and password field to be prefilled but that is not the case (if FF has stored only one user/pw combi for a specific URL than it automatically prefills this combi in your form)

Notice that I don't (want to) use cookies. I verified in FF password manager that username and password had actually been stored (they were)

Here's the code for this page:

<form name="login_form" id="login_form" autocomplete="ON" onsubmit="javascript:xajax_action_login(document.getElementById('user_name').value, document.getElementById('password').value); return false;">
    <div class="login_line">
        <div class="login_line_left">name</div>
        <div id="user_name_id" class="login_line_right"><input size="16" maxlength="16" name="user_name" id="user_name" type="text"></div>
    </div> <!-- login_line -->
    <div class="login_line">
        <div class="login_line_left">password</div>
        <div id="password_id" class="login_line_right"><input size="16" maxlength="16" name="password" id="password" type="password"></div>
    </div> <!-- login_line -->
    <div class="login_line">
        <div class="login_line_left"> </div>
        <div class="login_line_right"><input class="button" value="login" type="submit">
    </div> <!-- login_line -->
</form> <!-- login_form -->

What is wrong with my code? How can I get autocompletion to work in FF with my code?

Autocompletion does work correct with for instance gmail. Each time I visit the login page of gmail, the email and password fields are correctly prefilled. I don't use the 'remember me on this computer' checkbox so no cookies are used.

I would greatly appriciate your help.
Jasper

Update
Autocomplete is enabled in firefox. I want to stay IE compatible.