Just to clarify what's going on, if it isn't obvious...

Your "Buy Me a Beer!" link doesn't render properly on either browser.

In IE, the avatar IMG element is rendering a non-existent image file. It probably shouldn't render at all since I haven't selected an avatar.

In FF, the IMG element is correctly not rendering, but the entire poster's information block is rendering in the wrong place.

The avatar is still rendering with a red X.

Here's the problem. When simply viewing, the IMG element is rendering as:

<img alt='' src='/Data/Sites/1/avatars/blank.gif' />
 

When replying, the IMG element is rendering as:

<img alt='' src='http://www.mojoportal.com/Data/Sites/1/avatars/' />
 

Besides the fact that the reply form is missing the file name ("blank.gif"), note that the src URL uses the full absolute path rather than a relative path.