Most preference callbacks use literal strings for their domain filters, which
means that there's no need to make copies of them at all. Currently, however,
every preference observer node makes a separate heap-allocated copy of its
domain string.
This patch switches the domain string storage to nsCString instances, which
dramatically reduces the amount of unnecessary copies, at the expense of
making the callback nodes slightly larger.
MozReview-Commit-ID: 8NA3t2JS2UI
faabba2e6f