> 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.

I think Firefox focus does that on android, I'm sure there's a way to get the same result on a desktop with some flags and pointing to a config file (or a read only profile folder maybe?)