No idea what is causing that. The tooltip script is definitely the culprit. To try to narrow it down, I'd recommend creating a very simple page, like:
Code:
<html>
<head>
<title>test</title>
</head>
<body>
<form id="address_form">
Test:<br/>
<input type="text" name="blah" /><br/>
<input type="radio" value="1" name="address[Ship_Address_Is]" class="radio" />
<span>Residential<br></span>
<input type="radio" value="2" name="address[Ship_Address_Is]" class="radio" />
<span>Commercial</span>
</form>
</body>
</html>
Then, add your tooltip code to that page and see if it still happens.
Also, a few things to note:
1. On the address form page (and probably others, didn't check), you have Javascript errors occurring. You can see them in Firefox's Javascript console. It looks like this script is using JQuery, which can conflict with Mootools. The errors you are getting are related to Slimbox and Mootools, and are there because JQuery is setting the $ variable that Mootools relies on.
2. In your code for the address form, you had a <br> tag inside the <span/> tag for "Residential". You should not put that tag inside a <span/>. A span tag is an inline element. If you want a break there, either change that <span/> to a <div/>, or move your <br> so it is after the closing </span> tag.
3. For XHTML validation, I'd recommend changing that <br> to <br/>