Validate 2 fields. Working in IE, but not in Firefox

I have a page that has two fields(drop-downs) which are "Length of Service in years:"and the second field "Job Type:" with a  "submit" button right after.

In IE, if the fields are left blank, and  the SUBMIT button is clicked, then I get the two pop up boxes that remind user to fill in those fields before submitting the page. So it works in IE.

In Firefox, however, if I click the "Submit" button, then I only get one pop up box to remind user to enter the first field, but I do not get the second pop up for the second field. Also, when user clicks "OK" on the pop-up box for the first one, it automatically acts as though the Submit button is clicked and moves on to the next page.

Here's the code I am working with:

<SCRIPT>
function validate() {
if (main.los.options[0].selected) {
  alert('Please choose Length of Service in years.');
  event.returnValue=false;
}
if (main.job_desc.options[0].selected) {
  alert('Please choose Job Type.');
  event.returnValue=false;

}
}
</SCRIPT>

Here's the option value list for both fields:

 <td align="center">Length of Service in years:&nbsp;&nbsp;
              <select name="los" id="los">
                <option value ="0">Select</option>
                <option value="Less Than 6 Months">Less Than 6 Months</option>
                <option value="6-12 Months">6-12 Months</option>
                <option value="1-3 Years">1-3 Years</option>
                <option value="3-5 Years">3-5 Years</option>
                <option value="More Than 5 Years">More Than 5 Years</option>
              </select></td>

------

 <td align="center">Job Type:&nbsp;&nbsp;
              <select name="job_desc" id="job_desc">
                <option value ="0">Select</option>
                <option value="Administrator">Administrator</option>
                <option value="Director">Director</option>
                <option value="Physician">Physician</option>
                <option value="Nurse Director">Nurse Director</option>
                <option value="Assistant Nurse Director">Assistant Nurse Director</option>
                <option value="RN">RN</option>
              </select></td>

And lastly here's what I have for the SUBMIT button:

 <input name="" type="submit" onsubmit="validate()">

So how do I fix this?