Private mode data leakage in firefox

[ancipital]

Steps to reproduce:

Open a private browsing window. Set some temporary permissions within any scope to make the site work. Close the private browsing window, if you wish.

Actual results:

The temporary permissions needed to make the site work remained visible within the rules list of umatrix's dashboard, when viewed from the normal (non-private) window. These included items like the domain name, leaving quite a large trail.

Expected results:

The temporary rules from the private browsing instance should not be viewable outside of the private browsing instance- and should be automatically destroyed when the user exits private browsing.

This may well be a umatrix rather than firefox bug- I don't know whether the way that the addon shares data between private and non-private browsing is normal. Either way, addons not being sandboxed/invoked separately between modes may lead to further problems of this type in future.

(Also filed as https://bugzilla.mozilla.org/show_bug.cgi?id=1203923 for consideration of firefox devs)