Some researchers at Yale have been working on a project [1], due to be presented this weekend, that disables the FBI from gaining any meaningful information via Javascript exploits. The idea is to contain each "pseudonym" in its own virtual "nymbox", and only that one box. Read the paper for more, it's interesting. [1] http://arxiv.org/pdf/1312.3665.pdf
I also have an idea to stop the FBI from gaining any meaningful information via Javascript exploits: PGP or Ed25519 sign all .js files and have a browser that only executes signed code. Not only do you need to obtain access to the server, you also need the private key of its operator. (Which should be kept offline for signing.) If they don't surrender the key, then you cannot compromise their visitors. This can also…
Or am I missing something?