Make sure our bundled WebExtensions are running in Private Browsing Mode

We might need to flip a pref to allow our bundled WebExtensions to run in Private Browsing Mode starting with ESR 68. See:

https://bugzilla.mozilla.org/show_bug.cgi?id=1511636 https://bugzilla.mozilla.org/show_bug.cgi?id=1457001

for context.