Ronald, please check whether the problem persists after upgrading to 3.3.6. See the following tech post for more information:
http://www.mendix.com/tech-blog/mendix-business-modeler-4-4-3-and-3-3-6-have-been-released/
In short: the Firefox 18 fix present in 3.3.5 could lead to unpredictable errors in certain forms in all browsers. Version 3.3.6 fixes FF18 compatibility in a different way.