Your browser security settings don't permit automatic execution of paste operations

Your browser security settings don't permit automatic execution of paste
operations

I am using a MaskedEditExtender within my custom control in an ASP.Net
application. Whenever I try to paste some data into the textbox,
especially in firefox browser. A popup alert prompts me to enter "Your
browser security settings don't permit the automatic execution of paste
operations. Please use the keyboard shortcut Ctrl+V instead.".
I want to avoid this popup alert. Please provide your suggestions.
Thanks in advance, Santhosh