Unfortunately, when developing a complex web application like Unbounce (specifically our WYSIWYG landing page editor), we had to choose between speed of development and browser support. In order to increase our product capabilities to support feature demand we chose to leave IE out of the process for the time being.
Rest assured that the pages that Unbounce publishes work fine in all reasonably current browsers including IE.
Firefox and Chrome are both free downloads and you can continue to use IE for your normal browsing while switching to one of the others to use Unbounce for work purposes.
We understand this isn't a perfect answer, but we have found that it's currently the best way to maintain and progress rapidly with the product for the bulk of our customers.