I had this problem. It happened in all my Firefox
profiles, some of which didn't have any extensions. It didn't happen with Safari, ping worked OK, etc - nothing to do with the router or network.
In a clean (no extensions) Firefox profile, I disabled all the multimedia plugins - Flip4Mac
, Quicktime, ShockwaveFlash and Silverlight
. Re-tried Facebook, and it worked. Re-enabled the above plugins one by one to see which one it was - it still works now, with all four enabled. Cleared caches etc, still works.
So I figure somewhere, some content has caused one of the above plugins to save some state which prevents it working, and that some advert on facebook's main page uses said plugin to stream some content. Turning the plugins off and on again has presumably reset the plugins configuration state to default.
It's possible that whatever corrupted one of these (notoriously vulnerable) plugins might have been an attempted exploit, so I wouldn't advise ignoring the problem.
Recommended fix, given network is OK and problem is actually with Firefox:
1) Disable all multimedia plugins
2) re-enable or, preferably, update them with new ones, sourced independently
(by independently I mean not via their own update checkers - chances are,
any exploit is trying to replace the plugin with a zombie of some sort.)