Changing classes to `final` avoids complaints about deleting a class with
virtual functions. Making destructors private avoids static_asserts from
XPCOM code about refcounted classes with public destructors.
MozReview-Commit-ID: IiPrZln1wvj
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extra : rebase_source : 5d70eea909e9d8c7f191c465b700b778b7ff564c