You need to update the roll-up of your organization to at least 12 (the latest at the moment). I'm surprised, though, that you got as far as to actually getting the error. You shouldn't be able to log in and display the "main" forms when using unsupported browser.
EDIT:
It's definitely not an issue with RU12 and browser brand. I just created a new organization and added the following functionality via a web resource.
function disableFullNameOnLoad(){
Xrm.Page.getControl("fullname").setDisabled(true);
alert("Full name disabled on load.");
}
function enableFullNameOnSave(){
Xrm.Page.getControl("fullname").setDisabled(false);
alert("Full name enabled on save.");
}
As the nomenclature implies, the disabling is performed in the form's onload and enabling in its onsave. I entered the CRM (RU12, OP) using FF, IE and Cr. The behavior was consistent - greyed-out fullname as I open the form and editable when I save it.
So whatever issue there's on your platform, it's not directly related to cross browser incompatibility. Can you check again that you get the discrepancy in behavior between IE and FF when you add the exact code from my examples, connected to the events exactly the way I described, please.