If your form needs to call some custom code onsubmit, check TMT Validator's docs for additional details.
In this case you also have to pay attention to the order in which the .js file get included. Due to some exotic IE's quirks, you have to include tmt_ajaxform.js, then tmt_validator.js. In all other cases, and for FF users, the order just doesn't matter.
Please note Ajaxform requires TMT Validator version 2 or above, older releases aren't compatible.