Although one major disadvantage I've discovered recently is that if you embed an iframe (with a no-cache response header, i.e. Facebook's like button) it stops Firefox adding the page to the back-forward cache. Same with Webkit if the protocols don't match. https://developer.mozilla.org/en/using_firefox_1.5_caching

I wonder how much extra bandwidth Facebook has wasted worldwide by doing this? Twitter allows caching for 30 mins at least, good enough for back/forward caching to be useful.