Fixed with revision 24790
(Now the Tabulator extension is compatible with Firefox 3.0!)

The reason for this problem:
Firefox 3 has introduced this security restriction to avoid attackers from 
exploiting vulnerabilities in extensions. It essentially restricted the ability 
of web content to access “chrome://” resources. See bug 428848 [1]

The solution:
A new chrome manifest flag - contentaccessible. 
However this flag is not backward compatible. Therefore there are two content 
directives in the chrome.manifest 

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=428848