Iframes in Google Sites are not displayed in Firefox.

Google sites are running in SSL-encrypted mode (https://)

If you are trying to add an http-Iframe on a SSL-encrypted website (https://), it won't work any more since Firefox 23 because Mozilla has decided to blocked all unencrypted content on encrypted website.

You can change this behaviour in your own Firefox installation by changing the settings in your Firefox browser.

• Type in the address bar: 

about:config

• Browse to the following line:
  

  security.mixed_content.block_active_content

• By default this setting is set to true. Toggle this setting to:
  

  false

• Close the settings tab

Now your http-Iframes should work in your SSL-Encrypted web-site.