Some sites do not work when FirePHP is installed and enabled. These sites fail when matching the User-Agent string the browser sends since FirePHP appends "FirePHP/x.x" to this string.
This is not so much a problem with FirePHP than it is with some sites since any modified User-Agent string will likely break these sites.
Future versions of FirePHP will allow you to enable and disable FirePHP on a per-site basis to get around this problem.
In the meantime you can globally disable FirePHP when visiting these sites by using the "Tools" -> "FirePHP" menu.
