JQuery :: Custom Selectors Are Ignored In FF4b10 And Chrome?

I wrote a simple custom selector, that should allways be 'true'. Here's the code:

jQuery.expr[':']['first-letter'] = function(){ return true; };

Now the problem:

Firefox 3.6: $('*').is(':first-letter') -> 'true' // that's what I expected
Firefox 4.0b10: $('*').is(':first-letter') -> 'false' // why ???
Chrome 9: $('*').is(':first-letter') -> 'false' // why ???

I've made these tests with jQuery 1.4.4 and 1.5 - nn difference.