jQuery's ajax is causing a full page refresh in FireFox

$.ajax({
async: false,
type: "POST",
url: "Default.aspx/DoSomething",
data: "{" + parms + "}",
contentType: "application/json; charset=utf-8",
dataType: "json",
cache: false,
success: function(data) { succesfulPost(data); },
error: function(XMLHttpRequest, textStatus, errorThrown) { errorPost(textStatus, errorThrown); }

<input type="submit" onclick="doAjaxSubmit();return false;" value="Update" />