Strange collapsible text behavior in FireFox

I am using Panels with collapsible text for the YouPlatform front-page (http://www.youplatform.com) and am getting very strange visual displays when viewing my site using FireFox, but not IE.

The behavior is as follows: when I load the front page, click on collapsible text and then scroll back up, I get a series of lines drawn all over the text (I've attached a screen shot below). The number of lines varies according to the speed at which I scroll back up, with more lines being drawn the slower I scroll up.

Has anyone seen this sort of behavior before? I can't tell if it's coming from Panels or Collapsible Text.

Thank you for your help.
-YouPlatform.com

In case it's helpful, I've also included the code for one of the nodes being displayed in the panel:

<script type="text/javascript" src="/misc/collapse.js"></script> 
<img src="images/How-To-Buy.jpg"><br clear=all>
<img src="images/Buyers---Step-1.jpg"><br clear=all>
<fieldset class="collapsible collapsed">
<legend>Read more</legend><ul><li><b>Click on the Browse All Services Offered tab</b> at the top of any page.<img src="images/ClickOnTab.jpg"><br clear=all></li><li><b>Filter services </b>by the type of service you're looking for ("Essay Editing" for example). If you can't find the service you're looking for try the "Marketplace - All Other Services" category. <img src="images/Service-Type.jpg"><br clear=all></li><li><b>Find the right service for you. </b>You can sort through services based on many features including its overall rating, price and by through reading reviews written by others. <img src="images/ViewProviders.jpg"><br clear=all></li></ul></fieldset>
<img src="images/Buyers---Step-2.jpg"><br clear=all>
<fieldset class="collapsible collapsed">
<legend>Read more</legend><ul><li><b>Pay for your services. </b>After adding a service to your cart you can pay using any major credit card or PayPal. You will receive an email after your transaction is processed. <img src="images/Payment.jpg"><br clear=all></li> 
<li> <b>Receive your services. </b>After payment is completed, the service provider(s) you selected to work with will be notified by email. They will contact you by email to schedule a time to work with you within the next seven days (unless otherwise specified in the service description). If you do not receive your service within this timeframe for any reason you can email youplatform@gmail.com for a full refund. Please reference your order number. <img src="images/Order.jpg"><br clear=all></li>
</ul></fieldset>
<img src="images/Buyers---Step-3.jpg"><br clear=all>
<fieldset class="collapsible collapsed">
<legend>Read more</legend><ul><li><b>Rate and review your provider. </b>After your receive your service, your provider will mark the transaction as complete. You will then receive an email asking you to review the quality of the service you received. Please take a moment to rate the service and share your thoughts; this really helps out other people using YouPlatform and makes this community stronger.<img src="images/Review.jpg"><br clear=all></li>
</ul></fieldset>