this.query.el.removeClass('atwho-query').addClass('atwho-inserted').html(content).attr('data-atwho-at-query', "" + data['atwho-at'] + this.query.text).attr('contenteditable', "false"); on line 835 in your js file is the .attr('contenteditable', "false"); part stops the elements from being removed via backspace you can only remove them when you select the entire block and delete it then in firefox.
this.query.el.removeClass('atwho-query').addClass('atwho-inserted').html(content).attr('data-atwho-at-query', "" + data['atwho-at'] + this.query.text).attr('contenteditable', "false");
on line 835 in your js file is the .attr('contenteditable', "false"); part stops the elements from being removed via backspace you can only remove them when you select the entire block and delete it then in firefox.