Change Second Menu Depending On Selection In First

Basically I have a form, with two simple select menus, both identical except for different name values to post:

HTML Code:
<p>Make current?</p>
<select name="is_current">
<option value="yes">Yes</option>
<option value="no">No</option>
</select>
<br />

<p>Put in list?</p>
<select name="in_list">
<option value="yes">Yes</option>
<option value="no">No</option>
</select>

Some PHP code governs whether the "yes" or the "no" option in either menu gets the 'default' "selected="selected" showing, based on database information. For instance, in the first menu, if the page deals with a case where the the first "is_current" value is indeed current, then the PHP code will write the default "selected="selected" for "yes":

HTML Code:
<select name="is_current">
<option value=yes" selected="selected">Yes</option>
<option value=no">No</option>
</select>

If the page deals with a case where the value isn't current, the selected="selected" code will be written for the "no" value instead. The same goes for the second select menu. Now, the logic of my situation requires that, if the user selects "Yes" in the first "is_current" drop-down, then only "Yes" should be available to select in the second "in_list" menu. Anyway, I don't know javascript very well, but I found a function that seemed to be able to do this.

Code:
<script type="text/javascript">
(function () {
function updateLevel() {
var limit = Number(this.value);
// find the next select element
var selects = this.form.getElementsByTagName('select');
var select;
for (var i = 0; i < selects.length; i++) {
if (selects[i] === this && selects[i+1]) {
select = selects[i+1];
break; .....

(Apparently the script has to be put or called just before the </body> tag at the end of the page.) Then the select menus are recast as:

HTML Code:
<p>Make current?</p>
<select name="is_current" id="in_list">
<option value="1">Yes</option>
<option value="2">No</option>
</select>
<br />

<p>Put in list?</p>
<select name="in_list">
<option value="1">Yes</option>
<option value="2">No</option>
</select>

Now this appears to work, up to a point. If the user selects "Yes" for the first "is_current" selection, then only "Yes" becomes available for selection in the second "in_list" menu. A posted value of "1" represents "yes", and a value of "2" represents "no". Except, in Firefox, the javascript somehow overrules the default selected="selected" code. That is, the source code in the browser is correct, but the javascript draws it incorrectly.

For instance, say the database/PHP has drawn selected="selected" for the value "yes" in the second menu. Even though I can see that the source code is OK in the browser, the javascript will make the browser show a "No" selection as the initial default. This happens in Firefox, though not in Safari, and I haven't tested other browsers yet. This may be obvious but I'm not sure of javascript.