How to make Firefox builds1 17% faster2

Firefox build times can be reduced by caching the WebIDL binding code generation step using buildcache's Lua plugin system. A new Makefile change (Bug 2027655) passes buildcache as a command wrapper to the Python WebIDL codegen action, allowing it to hash inputs, cache outputs, and replay them on subsequent builds. A custom Lua wrapper (webidl.lua) tells buildcache how to identify the command, locate input .webidl files and Python codegen scripts, and enumerate output files. Benchmarks show warm builds dropping from 5m35s (no cache) to 1m12s with buildcache and the plugin active — compared to 3m21s for ccache and 2m49s for sccache. The technique is generalizable to other deterministic Python codegen steps in the Firefox build system.