cross browser testing of jsp & flex application

We have a web application that we are going to launch soon. It is built using Flex and JSP. We need it to work across as many browsers as possible, we need you to test it either manually or using software across many platforms as possible. I do not mean statuc webpage test , i mean dynamic tests of all functions on the site. Please describe how you intend to test the site across different platform and your previous experience. We do not intend to give you accress to the code, we just need you to test it as a user would with login details. We also need the site tested for security. Again please supply reference to your experience in this field and your reporting techniques. we will give you access to the application once we are convinced that you are the right person.