Fixes#10724
The selection is never touched in an `editor.read`, but BEFORE starting
an `editor.update` it is synced with `window.selection`. Firefox for
some reason loses the editor selection, so on the next update the
selection is null.
For reference, there was a brief discussion on the Lexical Discord
server:
https://discord.com/channels/953974421008293909/1333916489870348309