First draft of changes as discussed with Firefox
Preview people.
In order to allow testing/evaluating these changes,
the new advanced setting `uiFlavor` has been added.
Default to `unset`; and can currently only be set
to `fenix`.
The new setting takes effect at launch only. This
new setting is not to be mentioned in official
documentation for now.
This is ongoing work, not open to external feedback.
Raymond Hill