Right now we dex everything in Fennec in a monolithic heap. (We support dexing in java-build.mk, and that's monolithic too.) However, Android's dx tool has grown some interesting functionality. It's now possible to pre-dex library JARs, to incrementally dex code under development, and merge the results together. This ticket tracks moving Fennec's dx build rules to moz.build, and supporting the new incremental dex behaviour. This should give us a nice local developer win. In the short term, we should just experiment with using multiple threads when we dex. It's worth noting that these dex improvements aren't as valuable if we are also using Proguard.

We're not going to do this; Proguard is handled by Gradle, and --without-gradle is going to die soon: Bug 1414415.