The style attribute "-moz-user-input" can be used in FF to get the functionality  "contenteditable=false" working.
The value assigned defines if user input is accepted. the possible values are

none : The element does not respond to user input
enabled : The element can accepts user input. This is default
disabled : The element does not accept user input.

Eg : <asp:TextBox ID="uxFromDate" runat="server" style="-moz-user-input: disabled;"></asp:TextBox>// to disallow users to enter input
<asp:TextBox ID="uxFromDate" runat="server" style="-moz-user-input: enabled ;"></asp:TextBox>// to allow users to enter input


Refer to https://developer.mozilla.org/en/CSS/-moz-user-input