Form field contains spurious text when it should be blank

I have one form, for the field User expiry_page_url. The rails code
looks like this

    <div class='field'>
      <label for="user_expiry_page_url">Expiry page url - make sure you
include the full url, starting with 'http://'. If no url is entered,
the user will be sent to the default <%= link_to "demo_expired",
:controller => "user", :action => "demo_expired" %> page when their
account expires.</label>
      <%= text_field_tag 'user[expiry_page_url]', @user.expiry_page_url,
:size => 45 %>
      <span id="expiry_page_url_status"></span>
    </div>

and the generated html looks like this (for an example where the field
is blank for the user in question)

    <div class='field'>
      <label for="user_expiry_page_url">Expiry page url - make sure you
include the full url, starting with 'http://'. If no url is entered,
the user will be sent to the default <a
href="/user/demo_expired">demo_expired</a> page when their account
expires.</label>
      <input id="user_expiry_page_url" name="user[expiry_page_url]"
size="45" type="text" value="" />
      <span id="expiry_page_url_status"></span>
    </div>

My problem is, that if the expiry_page_url value is blank, then the text
'max' appears in the box. Now, this is my username for the site, and
what i'm suspecting is that firefox is remembering this (when i log in)
and for some reason shoving it into the user_expiry_page_url field, on
this edit_user page. It must be something happening after page load
because the value="" in the html.

Has anyone else had this sort of thing happening?

thanks
max