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Not aware it causes any security problems and if it's on a static page it isn't going to have any effect on band usage. If it's a live update page it'll use up some data but if you're on unlimited who cares. With most modern PCs a browser isn't going to make a huge hit on RAM or CPU. If you're using antique hardware however it may be worth shutting down, depends how much you're doing simultaneously and what the PC resources are.