> There are plenty of addons that I want to use occasionally that require full data access. I generally do trust them...

Seconded, except I don't trust most add-ons and don't want to have to trust them.

I want an easy way to launch a disposable browser session in any browser, totally isolated, with add-ons chosen (and downloaded) at launch time, and then erased of with the rest of the session when its last open page is closed.