throbber destination???

I added a text box to the prefs dialog to allow me to change the 
throbber destination.  I just upgraded to 1.3 and I noticed that it isnt 
working anymore.  It will take the change and save it and work fine but 
the next time I go into the dialog and go to that page, it clears the 
textbox.  I know I can just modify in the about:config but was wondering 
if anyone could lend a suggestion as to why the code isnt working 
anymore.  Here is how I do it:

<label value="Throbber Destination URL" control="throbberpage"/>
<textbox id="throbberpage" type="autocomplete" flex="1" 
searchSessions="history" timeout="50"       maxrows="6" 
preftype="localizedstring" prefstring="browser.throbber.url"/>

Any input would be much appreciated!
-- 

Jason LaBumbard
Software Engineer
Visual Automation