Firefox stops reponding in C# when we fileupload control

Question

When user is submitting forms and he gots message 'prevent this page from creating additional dialogs'.And if user checks checkbox then browser(firefox) stops responding .
This Form runs on Chrome very well.
In debug I saw that no source available for AsyncFileUpload.cs
What may be cause ?...Solution please help
Can I prevent this message
If not then I already put server side validations but even after then browser (firefox)stops responding .
Thanks in advance...