Firefox - Safari Return Opposite 'disabled' Values On Startup

This has me completely stumped. I have a multiple select form element in my HTML document that needs to be manipulated by two different sets of context-sensitive controls. One set of controls is marked up as follows:

<div id="divControls1" name="divControls1" style="display:none">
<form id="fControls1" name="fControls1">
<input type="button" id="btnAdd" name="btnAdd" value="Add" onClick="addStuff();" />
<input type="button" id="btnEdit" name="btnEdit" value="Edit" onClick="editStuff();" disabled />
<input type="button" id="btnDelete" name="btnDelete" value="Delete" onClick="deleteStuff();" disabled />
</form>
</div>

When I load this page into Safari (on Mac OS X) and set the style of divControls1 to "display:block," I have an enabled "Add" button, a disabled "Edit" button, and a disabled "Delete" button, just as I expected. (I monitor selections in a multiple selection element to turn the buttons on and off.) But when I load this page into Firefox (also Mac OS X), all three buttons are disabled at startup. My page runs a function called startup() when the body fires onLoad. To try to troubleshoot the problem, I wrote this line at the beginning of the startup() function:

function startup() {
alert (document.getElementById("btnAdd").disabled);
...

When I run this code in Safari, the alert returns "false" (not disabled), just as I would expect, and intended. However, the same code in Firefox (Mac OS X) returns "true" (disabled) ... but the same code in Firefox (WinXP) returns "false"!