Clear the cache and the cookies from sites that cause problems.
"Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
"Remove the Cookies" from sites causing problems: Tools > Options > Privacy > Cookies: "Show Cookies"
You can also try to clear the Flash cookies (LSO) and settings.
Flash Website Storage Settings panel:
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See also Flash Local storage settings:
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BTW: Do you only experience this problem with Firefox?