[kevingadd]

One big gap is that release builds of Firefox tend to have had profile-guided optimization applied, among other things, so you need to make sure you take steps like that correctly or your custom build will end up slower.

It's probably possible to produce a custom build for your machine that runs faster, whether by using custom instruction sets, or feeding different data to the optimizer when it's doing PGO. However, the risk you run there is that this could introduce bugs - Firefox and Chrome both regularly run into bugs caused by the optimizer or linker, and in some cases the fix is to stop the optimizer/linker from fiddling with that code.