ProcessRequest method of ashx file is not getting called

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VB

ASP.NET

Hi..!

I have a captcha for verification in my aspx page.
the code for captcha creation is written in a seperate class and calling that function from processrequest method of ashx file.

i have a imagebutton server control for captcha refresh.

every thing is working fine in chrome and IE. but captch refresh is not Working in FF26.0. The reason for this is, process request() is not getting called on postback.
why is this happening only in FF?

Could some one help me to get out of this..!

Thank You..!