[BluffTHIS!]

Sometimes there are errors and a pic just doesn't load. But are you sure it's not because they were ads of some kind blocked by an ad filter? Also firefox can be a little quirky sometimes and you need to exit it and then check your running processes to make sure it is fully killed and then restart it. Additionally, it could be because certain sites have such high traffic that the response time is greater and it all doesn't get loaded.