Use Tor Browser 10.5 as a watershed update
In #40042 (closed) @mcs raised the question about whether we need a watershed update before migrating to 78esr. We decided that wasn't needed, but we may want a watershed update for Tor Browser that would allow simplifying some of our implementations.
Some of the code areas that could be simplified described in that ticket. Let's continue investigating it here, and we should create individual tickets for each component that needs changes.
(I'm not sure how we can best document this, so I'm beginning with ticket numbers)