Viewstate issue in IE - but not in Firefox???

I don't think I can explain this quickly.
I'm using a Master page for the application I'm working on.  This Master page has an asp.net checkbox on it.  This checkbox has "autopostback" set to False.  This checkbox is only intended to be used as a flag to keep track of things on the client side.  When this checkbox has changed, I make an AJAX call to a page on the server.  Everything at this point works fine.  When navigating to another page, the code-behind of the Master page checks the a user profile flag and sets the checkbox accordingly.  The AJAX call updates the profile flag, so there is no need to perform a postback.  This works fine in IE and Firefox.
I also have pages with a Gridview on them.  You can change the pagination and sorting on this gridview, which require a postback.  In IE, when I navigate to a page with this Gridview, change the checkbox and then perform ...