When browsing http://moodle.site/userpix/index.php on a 1.8.3.+ install, I get the following error with Firefox 2.0.11 on Linux:

XML Parsing Error: not well-formed
Location: http://localhost/moodle-18/userpix/index.php
Line Number 100, Column 69:<a href="http://localhost/moodle-18/user/view.php?id=2&course=1"title="Admin User"><img src="http://localhost/moodle-18/user/pix.php/2/f1.jpg" style="border:0px; width:100px; height:100px" alt="Admin User" /></a>

The error is due to a missing whitespace between the trailing " character of the 'a' tag and the beginning of the 'title' tag. The same problem is present in 1.9beta2, although Firefox show no XML parsing error unless you set Administration >> Server >> Debugging >> XML Strict Headers.

The attached patches fix the issue for 1.8.3+ and 1.9beta2.

Saludos. Iñaki.