I found something that works in both FF and IE, and no "error on page" msgs. It sets an ID for both the search form and the text box. For example:
In search.tpl:
Code:
<form method="get" action="search.php" name="search" id="formid">
...
<input type="text" name="whatever" id="textboxid"
value="{$searchstring|escape:'html'}" onFocus="this.value=''"
tabindex="1" />
...
</form>
In home.tpl:
Code:
<body onload="document.forms.formid.textboxid.focus()">