Previously on Windows, where we don't have /bin/mv, we were recursing
into the data directory and copying files individually, which is very
slow, so automatic migration was disabled. Instead, try moving
directories with OS.File.move() with the `noCopy` flag. Moving
directories is technically unsupported by OS.File, but probably only
because of the possibility of a cross-volume copy (which is only
implemented for some platforms), and using `noCopy` hopefully prevents
that. If someone does have their data directory or storage directory on
a different volume, the migration might be quite slow, but leaving a
data directory behind in the Firefox profile directory (where it can be
easily misplaced with a seemingly unrelated Firefox reset) is worse.
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