We had a strange issue with Firefox 3.x and the access to document.styleSheets[x].cssRules (where x is a valid index in the styleSheets array).

It seems that if that property is accessed after the document.domain property is set then a security exception is throw:
 [Exception...  "Security error"  code: "1000" nsresult: "0x805303e8  (NS_ERROR_DOM_SECURITY_ERR)" ... ]
You can find a bug report here (the bug has a strange history as it starts as a quicktime related issue)

This is a test case that reproduces the issue:

How is this related to Lightstreamer?
usually on production environments you'll have the following Lightstreamer-related code:

during the bind call the domain set in the setDomain call is applied to the page, thus if after that call you try to access var cssRules=document.styleSheets[x].cssRules then you'll have an exception.

A work-around is to store the cssRules object before calling the bind method; doing so you should be able to access its properties even after such bind call.