Clippings and Delicious Post don't appear to be affected.
On Firefox 20, the Panic Button toolbar button icon is not displayed.
Send Tab URLs
Firefox 20 further optimizes the loading of browser tabs by not loading their content until the user clicks on the tab. This includes the web page title, which is used by Send Tab URLs to create the tab list. The problem is that if the browser tab isn't loaded, Send Tab URLs is unable to retrieve the title of the web page in the browser tab; it will appear as "null" in the URL list.