Writing bridge settings is broken in esr78-based Tor Browser

When trying to select bridges on about:preferences#tor I get

JavaScript error: chrome://browser/content/torpreferences/torPane.js,
line 666: TypeError: bridgeSettings is null

and the the respective bridge selection drop-down box/button gets greyed out.

@acat: I assume this is a rebase error and I'll therefore mark this ticket as a child of #33533 (closed).