Their https://www.bitfinex.com/pages/tos clearly states that "you agree to abide by any relevant laws in your jurisdiction, including reporting any trading profits for taxation purposes". This means that you must take personal responsibility and report your trading profits to your local tax authority. They will not do this for you, you must do it yourself.

Trading profits are subject to your local capital gains tax. This means that you must pay the capital gains tax on your trading profit - just like you would if you were trading EUR/USD. You can also deduct any losses you had. All that money you obviously had at MtGox when they closed down would, for example, be a deductible loss. You agree to comply with your local tax laws in the Bitfinex TOS.

It also says that "Suspicious transactions will result in a Suspicious Activity Report being submitted to the relevant regulatory and compliance bodies applicable in the registered members' verified country of origin.". This means that you will be reported if you are the money launderer who is doing the suspicious transactions. You also agree to this in their TOS.

Your question indicates that you do not want to follow your local fascist governments laws. That would be against Bitfinex TOS and you should not use it if that is the case.