XPCOM
  history, including stuff we don't use much anymore
  binary drop-in libraries, compiler independent
    - reference-counting for ownership
      - how reference counting works
    - QueryInterface for dynamic_cast
      - how query interface works, and is used
  component is an XPCOM object (?)
   - multiple languages
  interfaces and IIDs
   - NS_GET_IID should only be used on template parameters
       https://github.com/mozilla/gecko-dev/blob/06944cc30e4232e11d813b997e2048a30e52a250/layout/style/nsComputedDOMStyle.cpp#L2406
       https://github.com/mozilla/gecko-dev/blob/99ef95d12ff4ff66b5685691dc291ba4aaa38732/layout/style/nsComputedDOMStyle.cpp#L3570
       https://github.com/mozilla/gecko-dev/blob/301f982c2116554821cad946db98a5a7296e95dd/layout/style/nsComputedDOMStyle.cpp#L4523
  createInstance / getService (lack of enforcement)
  modules, factories
  contract IDs, CIDs, categories
  cycle collection
  performance costs of cross-library calls and virtual function calls
   - other abstraction costs
  dangers of drop-in binary components breaking the browser
  costs of maintaining binary compatibility
   - didn't get automatic modularity

String
  goals:
   - encapsulate memory management
   - avoid buffer overruns
   - avoid string copies (what's special about our string classes)
  1-byte vs. 2-byte, encodings
  dynamic vs. static typing
  Substring vs. String (and AString)
  ownership types:
   - reference-counted buffer
   - owned buffer
   - dependent buffer
   - AutoString
  binding references to temporaries as recommended pattern, e.g., with Substring, StringHead, StringTail; maybe more in future
  a + b creating temporaries
  buffer growth by doubling; exact at first better
  encoding conversion helpers