I know that mercurysalts are often being used in the production of MDMA. I wonder if it is possible that frequent use of MDMA could lead to an accumulation of dangerous levels of mercury in the body. If that would be the case, then it would probably be a realy bad idea to consume more than a certain amount of MDMA throughout your whole life.

Eat less fish.
According to This Study MDMA tablets were found to contain .050 to 1.230 micrograms of mercury per gram of MDMA tablets. The US EPA finds up to 145 micrograms of mercury in a 6 ounce can of tuna.
A can of tuna = 118 grams of MDMA tablets.

Its possible to get a particularly mercury contaminated batch of MDMA. Its far more likely to get tablets containing meth or any number of poorly studied synthetic drugs that are not MDMA.
I'd be more concerned about the identity of the drug than the trace quantities of mercury.

I'm not sure if I agree with the title of this video either, it seems to imply that all synthetic drugs contain mercury and are therefore bad. Lets not practice the appeal to nature fallacy which is already somewhat common on this forum.