Yes, I get the same message... but it's not an error, it's just a warning :) And this particular warning tells us that Firefox is not aware of jQuery selectors, at least not of the :gt() selector, but, as long as you know that it is a valid selector (and it works), you should just ignore the warning....
Another question would be, why Firefox examines the selector string inside the $() function in the first place.... I mean, we could write $ ("my super selector") and Firefox should not throw a warning, because $ () accepts a string as an argument and we gave it a string, so it should be fine.... well, jQuery will not know what to do with the string, because it's neither a selector string nor a HTML code string... but that's "jQuery's problem", and not "Firefox's"...