Check about:performance to see what is using so much RAM. Keep in mind that this is just your tabs and not firefox overall. If something looks fishy in there, you have your culprit. This won't tell you about ghost windows or any weird leaks. about:processes will give you a more detailed but still okay to look at picture.

How much memory is it using exactly? I would say up to 2 gb is normal. Anything above that is probably abnormal unless you're on some intense sites.

A more technical method would be to measure memory usage at about:memory.