Ahh ok.
Well, yes, Mercury can indeed be extremely toxic. It affects principally the brain, which is why such amalgams are not recommended for fillings in pregnant women and children under 6, adopting the precautionary principle. Not should it be used in that proportion of the population that have an allergy to mercury (thought to be around 2 in 100? something like that) It can also have an effect on the kidneys.
That expression "mad as a hatter" is thought to be derived from the occupational poisoning of milliners working with various mercury based compounds.
For everyone else - the toxicity is very much dependent upon the dose, and unless you have a mouthful of mercury amalgam fillings and were prone to bruxism you would still fall below the recommended maximum daily exposure.
We should be looking to minimise the use of mercury wherever possible on environmental grounds, but I think that for most people they will be getting a greater exposure to mercury from the regular consumption of fish and shellfish or even just the environment than from their fillings.