Well, for a while now, Firefox has been...almost lagging. I can't really describe it, but like, the page will load, but then it might say "not responding" and then it'll load. If I click the page while it's taking forever to load, it also says "not responding". To my knowledge, this is just Firefox doing this. Is it Firefox, or is it my internet?
Try to load the websites on another browser. This is the only way of knowing whether it's firefox or your internet. This is also one of the only ways to diagnose the problem.
Don't worry GG, many Firefox users are having the same problems since the latest update was released.
I've been noticing the exact same thing since i installed the latest update. I seem to get a lot more of the 'Not Responding' messages.
As other have stated you could try un-installing unused plug-ins, using CCleaner to clean your computer. I've tried all these to no affect but its worth a shot for you