Fix some comments, no functional changes.

This is currently an advanced config option as it is still experimental
and needs more testing.  When enabled it will copy the Firefox profile
directory into a tmpfs mount when the container is created, that is
discarded when the container is torn down.

The following directories are read-only bind mounted as usual:
 * profile.default/preferences
 * profile.default/extensions

Foot + gun options are bad, the "Get Addons" pane is force disabled via
a locked pref by virtue of being Mozilla + Google botnet bullshit, and
this commit also switches to an extension whitelist.

This seems to be the solution that the Tor Browser devs are going for,
which is sensible.

Loading Google Analytics as part of an IFRAME that implements an
internal `about:` URL, without being explicitly opt-in is the total
antithesis of privacy respecting.

I should have done this to begin with, and part of me wants to deprecate
all of these options entirely, but people will complain.

Only enable the workaround for the alpha series, and only if the version
is 7.5a2 or older, on the assumption that since it's fixed upstream it
will be reflected in the next Tor Browser alpha build.