its been a while since I made a Facebook App but I am pretty sure last time I checked you can made an app on a non-SSL domain and it will be iframed into the secure page, which in FF23 will break, so some apps may not work anymore. just saying.
If you access the app using https-ed Facebook, i.e. via https://apps.facebook.com/appname, it would use https:// for the iframe as well. So, no problems there (at least no new ones that did not already exist).
