I agree, but this presupposes a large comprehensive test suite giving you enough confidence to do such sweeping changes. I don't doubt Firefox has it, but most

I agree, but this presupposes a large comprehensive test suite giving you enough confidence to do such sweeping changes. I don't doubt Firefox has it, but most (even large, established projects) will not. A common case I've seen is that newer parts are relatively well covered, but older, less often touched parts don't have good coverage, which makes it risky to do such sweeping changes.