This question already has an answer here:
In this answer, suddenly I can't notify Ben Bitdiddle, typing "@B" won't show the autocomplete box. I have tried write "@Ben Bitdiddle" manual but after I enter, this notify words disappear (you can see yourself in this latest comment. Notify other people in other comment boxes on that same question still function. Refresh the page won't solve the problem. I use the latest Firefox version in Windoes 8.1.