This is the HTML:

 
<div id="sidebar">
<div class="<?php echo $tmq_sidebarwidth;?>">
<?php dynamic_sidebar( $tmq_pagesidebar ); ?>
</div>
</div>
<div class="clearboth"></div>

Everything is closed properly... And as mentioned, it works in everyone elses firefox. We all have the latest version - 23.0.1.

Why would this appear fine on someone elses but not mine?

moderator added tags to show the posted code

Maybe its using the ID in multiple places, have you done that check?

You may also want to try safe mode and see if it goes away.

• Use Troubleshoot Mode in Firefox

Thanks. I tried viewing it in safe mode but the problem is still there.

The ID only appears once but, again, why would the page load fine for other firefox users but not me?

Can you reproduce in a new profile, before trying that clear the cache.

• How to clear the Firefox cache

THEN

• Profile Manager - Create, remove or switch Firefox profiles

Do you have any add-ons, or use a proxy or filtering service, that might modify the structure of the page?

Did you verify that all tags are closed properly in the generated code?

Firefox closes and reopens tags if there is content that isn't allowed, so check source code and the generated code (Ctrl+A; View Selection Source) for any text marked in red.