So first we should try and figure out what actually is causing it since it may not actually be flash player itself, it could be something else interfering with it.
Have you tried one of the sites in another browser like Chrome or IE? Could be an update Firefox did, or an update one of your plugins did that caused it. Also if you updated the flash player recently, it could be possible the build you got isn't as stable as it could be and rolling back to an earlier one could fix it. So right now the idea is to get a better idea of what the cause is by ruling out factors, the easy one to rule out is if it's something related to firefox or not by trying it in another browser.
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Have you tried one of the sites in another browser like Chrome or IE? Could be an update Firefox did, or an update one of your plugins did that caused it. Also if you updated the flash player recently, it could be possible the build you got isn't as stable as it could be and rolling back to an earlier one could fix it. So right now the idea is to get a better idea of what the cause is by ruling out factors, the easy one to rule out is if it's something related to firefox or not by trying it in another browser.