Browser compatibility with IE and Firefox, chrome for design and JavaScript compatibility

Hi All

i am working with Asp.net web application in this it is giving some problems. those are with visual appearance and JavaScript.

The problems are in the IE7: i am using the menu control in the application but when i browse in the IE7 the menus are not supporting that

means the menus are not visible.

problem with Firefox browser is : when i am clicking on the Textbox/DropDownList we have to populate an confirmbox which shows an

message that "you want to change the values?"

then when we click on the ok button it will opens the drop down or allow to write text in that Text box in IE and chrome but not in FireFox

the JavaScript is  as follows

 function ChangeMgr(id) {

            if (!confirm("Do You want to change security level?"))

                document.getElementById(id).blur();

        }

and the dropdown tag is as follows

 <asp:DropDownList ID="ddlSecurityLevel" runat="server" OnSelectedIndexChanged="ddlSecurityLevel_SelectedIndexChanged"

                                        onfocus="ChangeMgr(id)">

                                        <asp:ListItem Selected="True">Preparer</asp:ListItem>

                                        <asp:ListItem>Office Manager</asp:ListItem>

                                        <asp:ListItem>Owner/Franchisor</asp:ListItem>

                                    </asp:DropDownList>

and the Textbox tag is

<asp:TextBox onfocus="ChangeMgr(id)" ID="txtMangerUserName" runat="server" OnTextChanged="txtMangerUserName_TextChanged"></asp:TextBox>

the above ChangeMgrId JavaScript is works fine in IE and Chrome but not in Firefox

these are the browser compatibility issues please check it and try to resolve these issues or if you find any links for this please send post it

The main Requirement is browser compatibility

how to make an application support all browsers types and versions for the CSS and JavaScript

Thanks to all in advance

Thanks And Regards 

V.S.R.Krishna