C# .NET - Your browser security settings don't permit automatic execution of paste operations.

I am using a MaskedEditExtender within my custom control. whenever I try to paste some data into the textbox, especially in firefox. 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.

I don't know that you can anymore.  Copy/paste via programmatic access has been exposed as a security threat.  I believe the browsers nowadays are looking to a confirmed human initiated action to have taken place to trigger the paste portion.