Background image retrieved from database not behaving properly in firefox browser

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ASP.NET

i have a div on my webpage and i am using cookies to store user data . i have made perticular button to open perticular div on my webpage. means i have set 12 buttons to open 12 different div on 12 different webpages connected with a single master page now my problem is:

when i click first button, image of that perticular button is retrieved from database
but when i click second button still first image from the database is retrieved. here i want second image to be retrieved .but once again when i click on the second button the second image is retrieved.

my problem is in the first click i get previous buttons image retrieved and for the second click i get what i want i.e my image of that perticular div. thus i have to click every button two times to retrieve the exact image . this is causing problem in mozilla firefox but on the other hand working nicely in IE and Chrome. pls help me out with this.