Stop FIREFOX from caching captcha

[mobilefor]

So I have this form here: http://www.piataterenuri.info/register.php The problem is that Firefox caches the captcha image and after an incorect input, or a page refresh the captcha will show the old image instead of the curent one.I've already placed:\[code\]header("Cache-Control: no-cache");header("Expires: Sat, 26 Jul 1997 05:00:00 GMT");\[/code\]And also changed the captcha img src to\[code\]<?php $rand=microtime() * mktime(); echo "src=http://stackoverflow.com/"captcha.php?time=$rand\""; ?>\[/code\]What else can I do to stop firefox from caching the image?