DropDownList PostBack problem

 Hi
I am facing with a strange problem (only on Fire Fox). I have some code in code behind in a try { }  catch { }.
In catch part there is:
 
catch
{
ScriptManager.RegisterClientScriptBlock(Page, this.GetType(), "onclick", "alert('Error !');", true);
}  When there is no exception everything works perfect but when  something goes wrong and 'catch part' fires then dropdownlist postback's stops working - I mean I can't select anything in dropdownlists. After postback everything works ok.
Furthermore when I switch (or open another window) to any other program and then back to Fire Fox everything works ok.

There is no such problem in Internet Explorer 7.

Can someone help me with this? ...