I'm pretty sure you have encountered a bug in Firefox 23. This will be fixed in Firefox 24, which is scheduled to be released Tuesday (Pacific time zone), barring any last minute issues. If you need to use this feature immediately, you can install the beta test version of Firefox 24 from the following page: [http://www.mozilla.org/en-US/firefox/all-beta.html] (choose your preferred language and operating system). Note that installing a beta version will put you on track to get upgraded to a test version of Firefox 25, so if the form feature isn't critical, it's easier to wait for the Tuesday release.